RFR: Move SATB flushing machinery out of shared code

[Roman Kennke]

Great!! Do it!
Roman

> http://cr.openjdk.java.net/~shade/shenandoah/satb-prompt-3/webrev.01/
> 
> We have the shared code hack for handling Shenandoah SATB flushes. While it seems generic for G1 and
> Shenandoah, I think G1 people want to implement these flushes differently. So, instead of
> upstreaming this part, we can just move it into Shenandoah-specific implementation of SATBMarkQueue.
> This reverts gc/shared/satbMarkQueue.cpp to upstream version.
> 
> Testing: tier3_gc_shenandoah, SATB-sensitive benchmarks, eyeballing SATB flush logging
> 
> Thanks,
> -Aleksey
>