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

[Aleksey Shipilev]

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