RFR: Refactor ShenandoahFreeSet

[Aleksey Shipilev]

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

Several improvements:
 a) Untangle from ShenandoahHeapRegionSet, in anticipation for merging with cset and other
facilities for centralized region handling;
 b) Rewrite humongous allocation path with a simpler code;
 c) Simplify unsafe_peek_free;
 d) Various style improvements;

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey