RFR: Tune heuristic defaults and behavior for improved stability

[William Kemper]

Also, some minor changes to logging.

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Commit messages:
 - Fix whitespace
 - Replace magic numbers with symbolic constants
 - Tweaks to heuristics

Changes: https://git.openjdk.org/shenandoah/pull/209/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=209&range=00
  Stats: 32 lines in 6 files changed: 14 ins; 5 del; 13 mod
  Patch: https://git.openjdk.org/shenandoah/pull/209.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/209/head:pull/209

PR: https://git.openjdk.org/shenandoah/pull/209