RFR: Parallel verification

[Aleksey Shipilev]

Hi,

Verification takes a while now, since we are verifying lots of floating garbage
(still marked, and can be walked by GC). I think we can trivially parallelize it:
  http://cr.openjdk.java.net/~shade/shenandoah/verifier-parallel/webrev.01/

We are deliberately not using work-stealing, because we don't want to rely on
ShTQ correctness, and not using usual ShHeap region iterators for the similar
reason.

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey