Use G1 GC.
Fixed deadlock.

The attempt to fix this problem from 3cecb36 was incomplete.  There was
another instance where timer.cancel() was being called but didn't need
to be.

I've removed some unneeded synchronization, which was causing a
deadlock,  and added a separate scheduler service for the MesosTracker
timers.
Improved handling of TaskTracker cleanup.
Added jobtracker metrics.