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

[Roman Kennke]

Ok.

Roman

> 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
>