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

[William Kemper]

Not clean, has follow up bug fix

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Commit messages:
 - 8350605: assert(!heap->is_uncommit_in_progress()) failed: Cannot uncommit bitmaps while resetting them
 - 8342444: Shenandoah: Uncommit regions from a separate, STS aware thread

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/171/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=171&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342444
  Stats: 515 lines in 9 files changed: 398 ins; 89 del; 28 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/171.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/171/head:pull/171

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/171