RFR: Move SATB flushing machinery out of shared code

[Aleksey Shipilev]

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