RFR: FreeSet refactor: bitmaps, cursors, biasing

[Aleksey Shipilev]

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

This is actually 3-in-1 patch, because there is a fair amount of cohesion between all of them. Brief
tour of changes:

 *) Allocation code moved from ShenandoahHeap to ShenandoahFreeSet. This will soon become the
ShenandoahHeapRegionManager, if you see where it goes. The API is cleaned up a bit, and privately
used methods are now really private.

 *) FreeSet now maintains the bitmap instead of collection of regions. This allows maintaining the
sorted view of free regions, which is important when asynchronous recycling works. Async recycling
may add regions in random order, and humongous allocation would be unhappy about that. The bitmap
also allows resuming the allocation at the "earlier" region once it is added, making footprint story
easier.

 *) FreeSet maintains two boundary cursors, "leftmost" and "rightmost", that bound the bitmap. This
serves as the optimization for allocation path, because it can look up either boundary and see the
free region there. We could, in principle, use the optimized bitmap scans in future to make the
allocation in ragged heap even faster.

 *) Two boundary cursors also allow _biasing_ the allocations to either end. Biasing application
allocations help to optimize allocation path even further, because we clean up the beginning of heap
most of the time.

Testing: hotspot_gc_shenandoah, benchmarks, eyeballing Visualizer

LRU running under Visualizer with this change:
  http://cr.openjdk.java.net/~shade/shenandoah/freeset-bitmaps/lru.mp4

Thanks,
-Aleksey