RFR: Utilize parallel workers for GC tasks during safepoints

[Zhengyu Gu]

The change allows Shenandoah GC to utilize parallel GC workers during STW safepoints, without the restriction imposed by the number of concurrent GC threads.

The main purpose of the change is to allow us scale up/down GC concurrent and parallel workers independently.

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


Tests:

- jcstress (fastdebug)
- Shenandoah GC test (fastdebug and release)
- SPECjbb (fastdebug and release)
- SPECjvm (fastdebug)


Thanks,

-Zhengyu