Introducing the next generation of the job executor. Faster by great margin. Still very experimental and untested.

Still a lot todos:
- Cleanup failed jobs thread
- Exclusive jobs are not handled
- Suspend is ignored
- Unit tests
- Docs
Reverted manually commit https://github.com/Activiti/Activiti/commit/1fd11705d65efd6f47382c0f0e356189f4c8563b : restoring original job executor as we're implementing the new, improved one (see previous commit)