fix: add protections against dataraces in socket engines, cluster timer and queues.

[fclivaz42]

As discussed, here is a smaller PR with less risky changes.
I have changed the following:

• epoll, where I removed an unused variable and added a lock where elements were removed from the fds vector
• kqueue, where I have added a lock similar to epoll
• queues, where I have changed two booleans to std::atomic booleans, added a safeguard for the cli variable and guards for the removals vector.
• timer, where I added a guard to an unguarded if statement.

I believe these changes are light enough to not cause any issues whatsoever, and have kept the riskier changes out of this PR for now while I do more testing on my end.

Code change checklist

• I have ensured that all methods and functions are fully documented using doxygen style comments.
• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
• I have not built my pull request using AI, a static analysis tool or similar without any human oversight.

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[egorpugin]

My simple echo bots are stuck in epoll even with 10.1.5 version.
Does this fixes exactly those issues?

[fclivaz42]

My simple echo bots are stuck in epoll even with 10.1.5 version. Does this fixes exactly those issues?

You could try to test run by applying this PR as a patch and compile/install the library then test it. Feedback would actually be very welcome!

[braindigitalis]

are you saying a simple test bot now gets stuck in epoll and didn't before 10.1.5?

[egorpugin]

10.1.4 and 10.1.5 got stuck like after several hours.
And bot seemed working on 10.1.3 (or earlier).

With this patch my bot has survived this night. Definitely a good sign.
I'll report in 1-2 days. If it will be online - my issue is gone 99%

[egorpugin]

My bot is still online.
Good PR.

[egorpugin]

Dead again now.
Don't know if this is discord, dpp or my program/build.

Thread 5 (Thread 0x7f003854b6c0 (LWP 342729) "exe"):
#0  0x00007f0038687902 in __syscall_cancel_arch () from /lib64/libc.so.6
#1  0x00007f003867bb9c in __internal_syscall_cancel () from /lib64/libc.so.6
#2  0x00007f003867c20c in __futex_abstimed_wait_common () from /lib64/libc.so.6
#3  0x00007f003867e8de in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libc.so.6
#4  0x00007f0038844e00 in __gthread_cond_wait (__cond=<optimized out>, __mutex=<optimized out>) at /usr/src/debug/gcc-15.2.1-7.fc43.x86_64/obj-x86_64-redhat-linux/x86_64-redhat-linux/libstdc++-v3/include/x86_64-redhat-linux/bits/gthr-default.h:911
#5  std::__condvar::wait (this=<optimized out>, __m=...) at /usr/src/debug/gcc-15.2.1-7.fc43.x86_64/obj-x86_64-redhat-linux/x86_64-redhat-linux/libstdc++-v3/include/bits/std_mutex.h:173
#6  std::condition_variable::wait (this=<optimized out>, __lock=...) at ../../../../../libstdc++-v3/src/c++11/condition_variable.cc:41
#7  0x0000558029545c1b in std::thread::_State_impl<std::thread::_Invoker<std::tuple<dpp::thread_pool::thread_pool(dpp::cluster*, unsigned long)::$_0> > >::_M_run() ()
#8  0x00007f003884e424 in std::execute_native_thread_routine (__p=0x55805a32ccd0) at ../../../../../libstdc++-v3/src/c++11/thread.cc:104
#9  0x00007f003867f3c4 in start_thread () from /lib64/libc.so.6
#10 0x00007f003870256c in __clone3 () from /lib64/libc.so.6

Thread 4 (Thread 0x7f0037d4a6c0 (LWP 342730) "exe"):
#0  0x00007f0038687902 in __syscall_cancel_arch () from /lib64/libc.so.6
#1  0x00007f003867bb9c in __internal_syscall_cancel () from /lib64/libc.so.6
#2  0x00007f003867c20c in __futex_abstimed_wait_common () from /lib64/libc.so.6
#3  0x00007f003867e8de in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libc.so.6
#4  0x00007f0038844e00 in __gthread_cond_wait (__cond=<optimized out>, __mutex=<optimized out>) at /usr/src/debug/gcc-15.2.1-7.fc43.x86_64/obj-x86_64-redhat-linux/x86_64-redhat-linux/libstdc++-v3/include/x86_64-redhat-linux/bits/gthr-default.h:911
#5  std::__condvar::wait (this=<optimized out>, __m=...) at /usr/src/debug/gcc-15.2.1-7.fc43.x86_64/obj-x86_64-redhat-linux/x86_64-redhat-linux/libstdc++-v3/include/bits/std_mutex.h:173
#6  std::condition_variable::wait (this=<optimized out>, __lock=...) at ../../../../../libstdc++-v3/src/c++11/condition_variable.cc:41
#7  0x0000558029545c1b in std::thread::_State_impl<std::thread::_Invoker<std::tuple<dpp::thread_pool::thread_pool(dpp::cluster*, unsigned long)::$_0> > >::_M_run() ()
#8  0x00007f003884e424 in std::execute_native_thread_routine (__p=0x55805a32dd60) at ../../../../../libstdc++-v3/src/c++11/thread.cc:104
#9  0x00007f003867f3c4 in start_thread () from /lib64/libc.so.6
#10 0x00007f003870256c in __clone3 () from /lib64/libc.so.6

Thread 3 (Thread 0x7f00375496c0 (LWP 342731) "exe"):
#0  0x00007f0038687902 in __syscall_cancel_arch () from /lib64/libc.so.6
#1  0x00007f003867bb9c in __internal_syscall_cancel () from /lib64/libc.so.6
#2  0x00007f003867c20c in __futex_abstimed_wait_common () from /lib64/libc.so.6
#3  0x00007f003867e8de in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libc.so.6
#4  0x00007f0038844e00 in __gthread_cond_wait (__cond=<optimized out>, __mutex=<optimized out>) at /usr/src/debug/gcc-15.2.1-7.fc43.x86_64/obj-x86_64-redhat-linux/x86_64-redhat-linux/libstdc++-v3/include/x86_64-redhat-linux/bits/gthr-default.h:911
#5  std::__condvar::wait (this=<optimized out>, __m=...) at /usr/src/debug/gcc-15.2.1-7.fc43.x86_64/obj-x86_64-redhat-linux/x86_64-redhat-linux/libstdc++-v3/include/bits/std_mutex.h:173
#6  std::condition_variable::wait (this=<optimized out>, __lock=...) at ../../../../../libstdc++-v3/src/c++11/condition_variable.cc:41
#7  0x0000558029545c1b in std::thread::_State_impl<std::thread::_Invoker<std::tuple<dpp::thread_pool::thread_pool(dpp::cluster*, unsigned long)::$_0> > >::_M_run() ()
#8  0x00007f003884e424 in std::execute_native_thread_routine (__p=0x55805a32df10) at ../../../../../libstdc++-v3/src/c++11/thread.cc:104
#9  0x00007f003867f3c4 in start_thread () from /lib64/libc.so.6
#10 0x00007f003870256c in __clone3 () from /lib64/libc.so.6

Thread 2 (Thread 0x7f0036d486c0 (LWP 342732) "exe"):
#0  0x00007f0038687902 in __syscall_cancel_arch () from /lib64/libc.so.6
#1  0x00007f003867bb9c in __internal_syscall_cancel () from /lib64/libc.so.6
#2  0x00007f003867c20c in __futex_abstimed_wait_common () from /lib64/libc.so.6
#3  0x00007f003867e8de in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libc.so.6
#4  0x00007f0038844e00 in __gthread_cond_wait (__cond=<optimized out>, __mutex=<optimized out>) at /usr/src/debug/gcc-15.2.1-7.fc43.x86_64/obj-x86_64-redhat-linux/x86_64-redhat-linux/libstdc++-v3/include/x86_64-redhat-linux/bits/gthr-default.h:911
#5  std::__condvar::wait (this=<optimized out>, __m=...) at /usr/src/debug/gcc-15.2.1-7.fc43.x86_64/obj-x86_64-redhat-linux/x86_64-redhat-linux/libstdc++-v3/include/bits/std_mutex.h:173
#6  std::condition_variable::wait (this=<optimized out>, __lock=...) at ../../../../../libstdc++-v3/src/c++11/condition_variable.cc:41
#7  0x0000558029545c1b in std::thread::_State_impl<std::thread::_Invoker<std::tuple<dpp::thread_pool::thread_pool(dpp::cluster*, unsigned long)::$_0> > >::_M_run() ()
#8  0x00007f003884e424 in std::execute_native_thread_routine (__p=0x55805a32e090) at ../../../../../libstdc++-v3/src/c++11/thread.cc:104
#9  0x00007f003867f3c4 in start_thread () from /lib64/libc.so.6
#10 0x00007f003870256c in __clone3 () from /lib64/libc.so.6

Thread 1 (Thread 0x7f0038b36800 (LWP 342725) "exe"):
#0  0x00007f0038687902 in __syscall_cancel_arch () from /lib64/libc.so.6
#1  0x00007f003867bb9c in __internal_syscall_cancel () from /lib64/libc.so.6
#2  0x00007f003867bbe4 in __syscall_cancel () from /lib64/libc.so.6
#3  0x00007f0038702855 in epoll_wait () from /lib64/libc.so.6
#4  0x000055802936372d in dpp::socket_engine_epoll::process_events() ()
#5  0x00005580292a2052 in dpp::cluster::start(dpp::start_type)::$_0::operator()() const ()
#6  0x00005580292a1d55 in dpp::cluster::start(dpp::start_type) ()
#7  0x00005580295a055e in main ()