RFR: 8342444: Shenandoah: Uncommit regions from a separate, STS aware thread [v4]

[William Kemper]

> Currently, Shenandoah uncommits regions from its control thread. The control thread is responsible for starting GC cycles in a timely fashion. Uncommitting memory from this thread may introduce unwanted delays in the control thread's response to GC pressure.

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

 - Improve comments
 - Do not notify uncommit thread when uncommit is forbidden

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Changes:
  - all: https://git.openjdk.org/jdk/pull/22019/files
  - new: https://git.openjdk.org/jdk/pull/22019/files/997360ac..05db9558

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=22019&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=22019&range=02-03

  Stats: 69 lines in 4 files changed: 58 ins; 1 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/22019.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22019/head:pull/22019

PR: https://git.openjdk.org/jdk/pull/22019