RFR: Refactor uncommit handling: react on explicit GCs, feature kill flag, etc

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/refactor-uncommit/webrev.01/

This refactors uncommit handling:
 - There is now a global option that disables uncommit: no need to set large delay to disable, which
also unties interaction between delay and AlwaysPreTouch;
 - Explicit GC (e.g. System.gc()) now uncommits everything: user wants the memory, user gets it
without delay;
 - Flag renames to group everything under ShenandoahUncommit

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey