RFR: Add task termination and enhanced task queue state tracking

[Zhengyu Gu]

Hi,

Please review this patch that enhances overall task queue and task 
termination tracking. This should help us to evaluate upcoming task 
termination optimization and gather performance data for upstreaming 
ShenandoahOWST.

- Added tracker for task termination.

- Moved queue statistics reporting into queue set, so it can be used for
   Traversal GC.

  - Moved queue statistics reporting from CM::finish_mark_from_roots to
    CM::shared_finish_mark_from_roots, so it will report queue statistics
    for full gc.

  - Added queue statistics tracking and reporting in Traversal GC.

Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/termination_tracking/webrev.00/


Test:

   tier3_gc_shenandoah with -Xlog:gc+task+stats=trace
   (fastdebug + release)


Thanks,

-Zhengyu