FLINK-39968 [core] Reduce redundant AtomicBoolean allocation in MemorySegment to improve construction performance

[YYKI]

What is the purpose of the change

Reduce redundant AtomicBoolean allocation in MemorySegment to improve construction performance

Brief change log

• Replace the member variable final AtomicBoolean isFreedAtomic with a primitive boolean isFreed;
• Add the synchronized modifier to the free() method to ensure the atomicity and thread safety of the free state judgment and modification;

Verifying this change

This change is already covered by existing tests, such as MemorySegmentTestBase.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (yes )
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

[flinkbot]

CI report:

• 28b61a2 Azure: SUCCESS

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