RFR: 8312117: GenShen: Preempt OLD marking more quickly when YOUNG triggers arise [v3]

[William Kemper]

> Have regulator thread wait until expected condition is met

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

  Remove arbitrary timeout for wait, use accessors for _mode

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/333/files
  - new: https://git.openjdk.org/shenandoah/pull/333/files/7319707c..639ced2a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=333&range=02
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=333&range=01-02

  Stats: 14 lines in 1 file changed: 0 ins; 6 del; 8 mod
  Patch: https://git.openjdk.org/shenandoah/pull/333.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/333/head:pull/333

PR: https://git.openjdk.org/shenandoah/pull/333