RFR: Cherry-pick upstream parallel safepoint cleanup (JDK-8180932 and JDK-8185102)

[Roman Kennke]

This change brings Shenandoah repos to what I've pushed upstream
regarding parallel safepoint cleanup, and reverts the parts that have
changed during review. Since the upstream patch does not actually
provide parallel workers (only a GC interface), I added some code to
Shenandoah that provides workers when specifying
-XX:ParallelSafepointCleanupThreads=2 or more.

There are some small diffs between upstream and Shenandoah still, but
this involves the merging of deflation and nmethod marking into GC
passes. I left them alone for now, until this is upstreamed too.

http://cr.openjdk.java.net/~rkennke/par-safepoint-cleanup-upstream/webrev.00/
<http://cr.openjdk.java.net/%7Erkennke/par-safepoint-cleanup-upstream/webrev.00/>

Good to go?

Roman