RFR: Dynamic worker refactoring

[Zhengyu Gu]

This patch refactoring dynamic GC worker code out of 
ShenandoahCollectorPolicy, which is overcrowd.

My tests never show any benefits of dynamic workers so far, and our 
customized code does not seem to outperform shared implementation, so 
lets revert back to shared implementation.

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

Test:
hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu