[java] do not require a lock and hash lookup for calling Thread.current() from the main thread
Merge pull request #2118 from jonasmalacofilho/development

[java] do not require a lock and a lookup for calling Thread.current from the main thread
[java] Mutex implementation
[java] Added Gc
[all] Preliminary threads tests (still not working)
[all] Working tests for Threads
merge with development branch