RFR: Enforce GC workers constraint

[Zhengyu Gu]

CMS and G1 enforce GC worker constraint:
   Parallel workers >= Concurrent workers

Shenandoah should probably do so.

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

Test:

   tier2_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu