RFR: Cherry-pick: 8213373: Bulk MarkBitMap clearing methods

[Roman Kennke]

Yes, this looks good. Thanks!

Roman


> http://cr.openjdk.java.net/~shade/shenandoah/cp-8213373/webrev.01/
> 
> This cherry-picks JDK-8213373 and cleans up obsolete Shenandoah code. It will be backported to keep
> Shenandoah code in sync, this would be safe as we own markBitMap code everywhere except current jdk/jdk.
> 
> Testing: tier3_gc_shenandoah {fastdebug|release}
> 
> Thanks,
> -Aleksey
>