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

[Roman Kennke]

Ok. Thanks!

Roman

> 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
>