RFR: Eagerly drop CSet state from regions during Full GC

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/fullgc-cset-eager/webrev.01/

While doing more humongous move testing in Full GC, I ran into the corner case when we can have
"empty" CS regions after the Full GC moves, while the code was expecting only the Regular regions.
It makes more sense to eagerly drop CS state from regions, and turn them to CSet -> Regular, and
CSetPinned -> Pinned, which is what *_bypass already does. This also simplifies region state logic
in Full GC.

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey