RFR: Move BiasedLocking and general mark preservation to prolog/epilog

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/nofwdptr-mc-marks/webrev.01/

Another this spotted in sh/jdk: preserved marks preparation/restoration is not at the same nesting
level in mark-compact. It is cleaner to have them near each other in more "general" prolog/epilog
sections.

Testing: hotspot_gc_shenadoah

-- 
Thanks,
-Aleksey