RFR: 8345423: Shenandoah: Parallelize concurrent cleanup

[William Kemper]

Not clean

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Commit messages:
 - 8345423: Shenandoah: Parallelize concurrent cleanup

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/172/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=172&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345423
  Stats: 233 lines in 13 files changed: 87 ins; 56 del; 90 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/172.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/172/head:pull/172

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/172