[ruby 2.4] Queue and other thread-sync classes moved under Thread

[headius]

See https://bugs.ruby-lang.org/issues/8919.

The various thread-synchronization classes like Queue and ConditionVariable now live under the Thread namespace. The top-level constants still exist, but until we move them we get warnings like this:

/Users/headius/projects/jruby/lib/ruby/stdlib/singleton.rb:136: warning: toplevel constant Mutex referenced by Thread::Mutex
/Users/headius/projects/jruby/lib/ruby/stdlib/digest.rb:6: warning: toplevel constant Mutex referenced by Thread::Mutex
/Users/headius/projects/jruby/lib/ruby/stdlib/monitor.rb:244: warning: toplevel constant Mutex referenced by Thread::Mutex
/Users/headius/projects/jruby/test/mri/monitor/test_monitor.rb:8: warning: toplevel constant Queue referenced by Thread::Queue
/Users/headius/projects/jruby/lib/ruby/stdlib/webrick/utils.rb:129: warning: toplevel constant Mutex referenced by Thread::Mutex
/Users/headius/projects/jruby/test/mri/thread/test_queue.rb:8: warning: toplevel constant Queue referenced by Thread::Queue
/Users/headius/projects/jruby/test/mri/thread/test_queue.rb:9: warning: toplevel constant SizedQueue referenced by Thread::SizedQueue

[headius]

Ref #4293