RFR: Assorted monitoring support fixes

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/monitoring-fixes/webrev.01/

Several fixes:
  a) Poll "committed" from ShenandoahHeap, instead of VirtualSpace that now tells wrong values
anyway, because we do commits/uncommits separately. Remove VirtualSpace from ShenandoahHeap, because
it is not needed anymore;
  b) Fixup collector "old"/"young" designations. There is no good way to represent concurrent
collections, so fold it into "old" along with Full STW GC.
  c) Update Metaspace counters on this path, otherwise we have wrong values for it, and
TestMetaspacePerfCounters fails.

Testing: hotspot_gc_shenandoah, eyeballing "jstat -gc" output

Thanks,
-Aleksey