RFR: Adaptive heuristic should take reserved memory into account

[William Kemper]

Before this change, the heuristic considered the memory available to both the collector and mutators. Since the collector memory is not available to mutators, they may experience an allocation failure before the heuristic could anticipate. With this change, the heuristic looks at the memory available to mutators only. This has the effect of making the heuristic tend to start cycles earlier, thus avoiding degenerated cycles.

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Commit messages:
 - Have all triggers look at available memory after adjusting for headroom and penalities
 - Heuristic should use freeset available if this is lower than heap available

Changes: https://git.openjdk.org/shenandoah/pull/147/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=147&range=00
  Stats: 29 lines in 1 file changed: 17 ins; 10 del; 2 mod
  Patch: https://git.openjdk.org/shenandoah/pull/147.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/147/head:pull/147

PR: https://git.openjdk.org/shenandoah/pull/147