RFR: 8335356: Shenandoah: Improve concurrent cleanup locking

[William Kemper]

Not clean, but conflicts were easy to resolve.

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Commit messages:
 - Backport b32e4a68bca588d908bd81a398eb3171a6876dc5

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/106/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=106&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335356
  Stats: 18 lines in 2 files changed: 13 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/106.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/106/head:pull/106

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/106