RFR (S): Rebuild free set after update-refs, ignore cset in heuristics when update-refs are enabled

[Aleksey Shipilev]

Hi,

This solves two obvious problems with update-refs.

First, update-refs does not rebuild freeset, while actually recycling the
regions and updating heap occupancy counters. This leads ShenandoahHeap and
heuristics to believe there is free space, but then free set abruptly depletes,
and runs us into full GC.

Second, heuristics that do not expect update-refs think we will reclaim cset
from the previous cycle, and thus overshooting the starting time, again, driving
us to full GC.

Both fixed here:
  http://cr.openjdk.java.net/~shade/shenandoah/update-refs-heuristics/webrev.01/

Testing: hotspot_gc_shenandoah, targeted benchmarks

Thanks,
-Aleksey