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

[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.

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Commit messages:
 - Check for safepoint when stopping (stopping thread is java thread)
 - Fix ridiculous typo
 - Merge remote-tracking branch 'jdk/master' into shen-uncommit-thread
 - Fix shutdown protocol
 - Take heap lock when uncommitting bitmaps, uncommit thread joins STS.
 - Little bit of cleanup
 - WIP: checkpoint before sync up
 - WIP: checkpoint

Changes: https://git.openjdk.org/jdk/pull/22019/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22019&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342444
  Stats: 319 lines in 6 files changed: 229 ins; 74 del; 16 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