RFR: 8341379: Shenandoah: Improve lock contention during cleanup

[William Kemper]

Clean backport.

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Commit messages:
 - Backport f5f0852f51d3dc1001bf3d68b89f4aab31e05e61

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/119/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=119&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341379
  Stats: 39 lines in 1 file changed: 32 ins; 3 del; 4 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/119.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/119/head:pull/119

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/119