RFR (S) 8222185: Shenandoah should report "committed" as capacity

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8222185

Fix:
  http://cr.openjdk.java.net/~shade/8222185/webrev.01/

Shenandoah resizes the heap by (un)committing the individual regions. Therefore, the true heap
capacity is the "committed" size, not the maximum possible capacity. Currently, gc logs misreport
the heap size, for example with -Xms1g -Xmx4g:

  GC(2) Concurrent marking 183M->183M(4096M) 2.243ms

Notice "(4096M)", which is not the actual memory taken by committed heap. With this patch, we
properly report the current capacity:

  GC(5) Concurrent marking (process weakrefs) 137M->137M(138M) 2.750ms

I had to rewrite current uses: "capacity" -> "max_capacity" in heuristics, and "committed" ->
"capacity" in monitoring code.

Testing: hotspot_gc_shenandoah, eyeballing gc logs

-- 
Thanks,
-Aleksey


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