RFR (S): Update ShenandoahHeapRegionSampling protocol

[Aleksey Shipilev]

Hi,

I would like to update the ShenandoahHeapRegionSampling a bit:
  http://cr.openjdk.java.net/~shade/shenandoah/sampling-update/webrev.01/

Changes:
  *) Record and maintain "recently allocated" region flag
  *) Steal more bits for region flags
  *) Rehash the flags order
  *) Coarsen region_size and used/live data to KB to make up for stolen bits
  *) Update the sampling frequency to 40ms (25 FPS)
  *) ShenandoahHR:is_pinned enabled to be read outside the safepoint

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey