Crystal 1.21

[spuun]

Bumps amqproxy to Crystal 1.21.0 and adapts the code to it:

• Time.monotonic → Time.instant
• Requires amq-protocol 1.3.1 for Crystal 1.21 compatibility. That version replaces AMQ::Protocol::Frame.from_io with AMQ::Protocol::Stream#next_frame, so both the client and upstream sockets are now wrapped in an AMQ::Protocol::Stream and read frames through
  it.

Bonus:

• Makefile: LDFLAGS is now platform-conditional — the GNU-ld hardening flags (--as-needed, -z relro, -z now, -pie) aren't understood by Apple's linker, so macOS gets -Wl,-dead_strip_dylibs instead and make works there again.