RFR (S) 8223980: Shenandoah: Refactor and fix ObjArrayChunkedTask verification

[Aleksey Shipilev]

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8223980

Fix:
  http://cr.openjdk.java.net/~shade/8223980/webrev.01/

Testing on old Solaris exposed funny bugs. We need verification done more consistently here. The
change is mostly equivalent, except the startup check moved to ShenandoahHeap where we know the heap
region addresses.

Testing: hotspot_gc_shenandoah; eyeballing failure messages after increasing {chunk|pow}_bits

-- 
Thanks,
-Aleksey