RFR (sh/jdk11): Split G1 SATBMarkQueue/Set and Shenandoah SATBMarkQueue/Set

[Roman Kennke]

In order to reduce Shenandoah's shared-code exposure in
shenandoah/jdk11, I propose to split the current mixed implementation of
SATBMarkQueue/Set and have Shenandoah have its own implementation. I
basically forked the current mixed impl (with some slight modifications
and renames) to shenandoahMarkQueue.hpp/cpp and reverted the G1 impl
back to its original state. This gets us rid of a significant chunk of
shared-code changes vs upstream jdk11u.

Webrev:
http://cr.openjdk.java.net/~rkennke/shjdk11-shenandoah-satb/webrev.00/

Testing: hotspot_gc_shenandoah

Can I please get a review?

Thanks,
Roman