RFR(S): Merge GC thread pools

[Zhengyu Gu]

Hi,

The patch merges parallel GC and concurrent GC thread pools.

http://cr.openjdk.java.net/~zgu/shenandoah/single-worker/webrev.00/

Test:

   Fastdebug: specjbb2015, specjvm Debry, Compiler.sunflow
   Release:   specjbb2015


Thanks,

-Zhengyu