RFR: 8264851: Shenandoah: Rework control loop mechanics to use timed waits

[William Kemper]

We use a similar mechanism in `shGenerationalControlThread` already. Using a timed wait allows the control thread to be woken up before the wait expires.

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Commit messages:
 - Add documentation and make new method private
 - Use timed wait/notify for more timely control thread reactions

Changes: https://git.openjdk.org/jdk/pull/27502/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27502&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8264851
  Stats: 28 lines in 2 files changed: 18 ins; 6 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/27502.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27502/head:pull/27502

PR: https://git.openjdk.org/jdk/pull/27502