RFR: Abandon mixed collections if all candidates are pinned

[William Kemper]

A new `WAITING_FOR_FILL` state has been added. This state is now entered when all the candidates for a mixed collection cycle are found to be pinned (which should be quite rare). This state permits a new old generation collection cycle to begin (which itself begins with making these regions parseable).

This change also adds a new diagnostic flag `ShenandoahCoalesceChance` which is the probability (expressed as a percentage) of abandoning all the candidates of a mixed collection. Any abandoned candidate regions will be made parseable.

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Commit messages:
 - Make sure all candidates really are pinned
 - Abandon mixed collections if all candidates are pinned

Changes: https://git.openjdk.org/shenandoah/pull/236/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=236&range=00
  Stats: 82 lines in 9 files changed: 68 ins; 5 del; 9 mod
  Patch: https://git.openjdk.org/shenandoah/pull/236.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/236/head:pull/236

PR: https://git.openjdk.org/shenandoah/pull/236