RFR: Verify global and local gc-state status

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/verify-gc-state/webrev.01/

I wanted to do this for a long time, and now the work in mark-compact requires the verification step
like this. It checks that gc-states in both thread-local caches and global in ShenandoahHeap are the
ones we expect.

Testing: tier3_gc_shenandoah

Thanks,
-Aleksey