RFR (XS) 8240511: Shenandoah: parallel safepoint workers count should be ParallelGCThreads

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8240511

See bug for rationale. The best fix seems to be just using ParallelGCThreads and ditching
Shenandoah-specific option altogether:
  https://cr.openjdk.java.net/~shade/8240511/webrev.01/

Testing: hotspot_gc_shenandoah, eyeballing gross pause times

-- 
Thanks,
-Aleksey