Fix DaemonThreadFactory - reuse single Java thread factory

* Move defaultThreadFactory call to DaemonThreadFactory#initialize and store
resulting factory in @java_thread_factory
* In DaemonThreadFactory#newThread, use stored factory to generate new threads
instead of creating a new factory each time

Author: obulkin

lib/concurrent-ruby/concurrent/executor/java_executor_service.rb

@@ -88,10 +88,11 @@ class DaemonThreadFactory
 
       def initialize(daemonize = true)
         @daemonize = daemonize
+        @java_thread_factory = java.util.concurrent.Executors.defaultThreadFactory
       end
 
       def newThread(runnable)
-        thread = java.util.concurrent.Executors.defaultThreadFactory().newThread(runnable)
+        thread = @java_thread_factory.newThread(runnable)
         thread.setDaemon(@daemonize)
         return thread
       end