raftstore: move the handling of `CompactedEvent` to split-check worker.

[LykxSassinator]

What is changed and how it works?

Issue Number: Close https://github.com/tikv/tikv/issues/18532

What's Changed:

As the issue https://github.com/tikv/tikv/issues/18532 mentioned, this performance bottleneck occurs when processing StoreMsg::CompactedEvent after RocksDB compactions, where updating region sizes in store.meta requires a global lock and linear traversal of all regions.

In the current implementation, the most time-cosumping part is calc_ranges_declined_bytes operation, which scales poorly with high region counts, causing significant Raftstore stalls during the mandatory post-compaction updates.

This PR moves the handling of StoreMsg::CompactedEvent out of Raftstore's StoreMsg processing by converting it to a SplitCheckTask::CompactedEvent and offloading it to the split-check worker. This change delegates the compaction event processing to the dedicated split-check worker thread.

Optimize the handling of `CompactedEvent` in raftstore by moving it to `split-check` worker.

Related changes

• [ ] PR to update pingcap/docs/pingcap/docs-cn:
• [ ] Need to cherry-pick to the release branch

Check List

Tests

• [x] Unit test
• [ ] Integration test
• [ ] Manual test (add detailed scripts or steps below)
• [ ] No code

Side effects

• [ ] Performance regression: Consumes more CPU
• [ ] Performance regression: Consumes more Memory
• [ ] Breaking backward compatibility

Release note

Optimize the handling of `CompactedEvent` in raftstore by moving it to `split-check` worker.

[LykxSassinator]

Can you add more details about the issue? Which part do you think slowed down the message processing, acquiring meta locks or sending messages to overlapped regions?

Updated

[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2025-06-20 04:16:39.631327192 +0000 UTC m=+418052.354506169: :ballot_box_with_check: agreed by v01dstar.
• 2025-06-20 05:56:37.467341789 +0000 UTC m=+424050.190520768: :ballot_box_with_check: agreed by overvenus.

[LykxSassinator]

/hold

[LykxSassinator]

/unhold

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: overvenus, v01dstar, zhangjinpeng87

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Needs approval from an approver in each of these files:

• ~~OWNERS~~ [overvenus,v01dstar,zhangjinpeng87]
• ~~components/raftstore/src/coprocessor/OWNERS~~ [zhangjinpeng87]

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[ti-chi-bot[bot]]

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In response to adding label named type/cherry-pick-for-release-7.5.

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #18579. But this PR has conflicts, please resolve them!

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #18580. But this PR has conflicts, please resolve them!

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #18581. But this PR has conflicts, please resolve them!