RFR: Ensure tasks use correct number of workers

[Zhengyu Gu]

Ensure that GC tasks follow parallel/concurrent worker protocol, use 
correct number of worker threads.

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

Test:
   hotspot_gc_shenandoah (fastdebug and release)



Thanks,

-Zhengyu