RFR (S) 8242101: Shenandoah: coalesce and parallelise heap region walks during the pauses

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8242101

We are walking heap regions multiple times during final mark pause. We also walk them during final
update refs pause without any parallelism. We should really fix those, to allow more regions per heap.

Webrev:
  https://cr.openjdk.java.net/~shade/8242101/webrev.01/

I had to drop shenandoah_assert_heaplocked_or_safepoint from update_watermark, because it expects
that we are called from VMThread at safepoint, and with this patch, we are not.

By my count, the whole thing saves about 50us in pauses with the default number of regions.

Testing: hotspot_gc_shenandoah, benchmarks

-- 
Thanks,
-Aleksey