RFR: 8342444: Shenandoah: Uncommit regions from a separate, STS aware thread [v6]

[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 one additional commit since the last revision:

  Allow commits initially

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Changes:
  - all: https://git.openjdk.org/jdk/pull/22019/files
  - new: https://git.openjdk.org/jdk/pull/22019/files/7007415a..a00945c2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=22019&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=22019&range=04-05

  Stats: 9 lines in 1 file changed: 5 ins; 4 del; 0 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