RFR: 8312117: GenShen: Preempt OLD marking more quickly when YOUNG triggers arise [v2]

[William Kemper]

> Have regulator thread wait until expected condition is met

William Kemper has updated the pull request incrementally with two additional commits since the last revision:

 - Use same precision for time checks as log messages
 - Regulator lock should protect the write to mode

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/333/files
  - new: https://git.openjdk.org/shenandoah/pull/333/files/fbc5de1f..7319707c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=333&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=333&range=00-01

  Stats: 4 lines in 2 files changed: 1 ins; 1 del; 2 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