RFR: 8321816: GenShen: Provide a minimum amount of time for an old collection to run

[William Kemper]

Young collections are allowed to interrupt old collections. Under heavy load, this may result in starvation of old collections - which may in turn lead to even more memory pressure on the young generation. Adding a minimum guaranteed amount of time for the old gc to make progress will help avoid this situation.

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Commit messages:
 - Merge remote-tracking branch 'shenandoah/master' into reduce-old-starvation
 - Merge remote-tracking branch 'shenandoah/master' into reduce-old-starvation
 - Experiment with a minimum old marking increment

Changes: https://git.openjdk.org/shenandoah/pull/371/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=371&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321816
  Stats: 20 lines in 3 files changed: 19 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/371.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/371/head:pull/371

PR: https://git.openjdk.org/shenandoah/pull/371