RFR [8] 2018-11-20: Bulk backports to sh/jdk8

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20181120/webrev.01/

This backports recent work to sh/jdk8u. Lots of things are skipped, because they make no sense in
8u. List of changes:

 [backport] ShenandoahMemoryPool should support usage threshold
 [backport] Compact heuristics adjustments
 [backport] Cherry-pick bulk MarkBitMap clearing methods
 [backport] Remove ShenandoahWriteBarrierCsetTestInIR flag

Testing: hotspot_tier3_gc_shenandoah {fastdebug|release}

-Aleksey