RFR: Full GC should compact humongous regions

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/humongous-moves/webrev.01/

This implements humongous regions moves during last-ditch Full GC. This allows Shenandoah to survive
humongous-fragmenting workloads, at the expense of potentially longer Full GC, when the alternative
is OOME. This mirrors the upstream G1 RFE [1]. New jtreg test is the example of such the workload,
and it OOMEs Shenandoah in seconds without this fix.

Testing: hotspot_gc_shenandoah {fastdebug|release}, new failing test

Thanks,
-Aleksey

[1] https://bugs.openjdk.java.net/browse/JDK-8191565