RFR: Assorted monitoring support fixes

[Zhengyu Gu]

Looks good.

-Zhengyu

On 08/30/2017 06:13 AM, Aleksey Shipilev wrote:
> 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
>