RFR: ShenandoahPushWorkerQueuesScope should subclass ShenandoahPushWorkerScope

[Zhengyu Gu]

To eliminate duplicate code for pushing worker scope.

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

Test:

   tier3_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu