RFR: Handle scenario that not be able to satisfy the request for active workers

[Zhengyu Gu]

Newly updated EvilSyncBug test reveals that it is possible 
workers->update_active_workers() might not be able to honor the request, 
e.g. it failed to create additional workers, due to memory or os 
limitations, etc.

Under those scenarios, we may get assertion failure (debug) or reserve 
wrong number of queues for workers (product), which could result hard to 
find errors downstream.

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


Test:
   tier3_gc_shenandoah (fastdebug and release)


Thanks,

-Zhengyu