RFR: 8376357: Parallel: Convert MutableSpace classes to use Atomic<T>

[Thomas Schatzl]

Hi all,

  please review these changes that convert `MutableSpace` classes to use `Atomic<T>`.

Testing: gha, tier1-5

Thanks,
  Thomas

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Commit messages:
 - 8376357

Changes: https://git.openjdk.org/jdk/pull/29427/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29427&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8376357
  Stats: 31 lines in 5 files changed: 4 ins; 8 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/29427.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29427/head:pull/29427

PR: https://git.openjdk.org/jdk/pull/29427