RFR: Abandon mixed collections if all candidates are pinned [v2]

[William Kemper]

> A new `WAITING_FOR_FILL` state has been added to the old generation. 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.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Improve description of new ShenandoahCoalesceChance diagnostic option

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/236/files
  - new: https://git.openjdk.org/shenandoah/pull/236/files/7be1a1db..84c8e99a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=236&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=236&range=00-01

  Stats: 6 lines in 1 file changed: 3 ins; 0 del; 3 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