RFR (S): Parallelize partial GC cleanup

[Aleksey Shipilev]

Hi,

Matrix and bitmap cleanups take significant time in partial GC. The obvious
solution is to parallelize it at this point, since we have mined out the
sequential improvements.

Fix:
  http://cr.openjdk.java.net/~shade/shenandoah/partial-recycle-parallel/webrev.01/

Improves partial GC times significantly under heavy allocation pressure:
  http://cr.openjdk.java.net/~shade/shenandoah/partial-recycle-parallel/perf.txt

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey