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

[William Kemper]

Have regulator thread wait until expected condition is met

-------------

Commit messages:
 - Add timeout for extra paranoia
 - Change log level to debug
 - Have regulator thread wait until expected condition is met

Changes: https://git.openjdk.org/shenandoah/pull/333/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=333&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312117
  Stats: 44 lines in 3 files changed: 37 ins; 0 del; 7 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