eventstore: fix checkpoint update race

[lidezhu]

What problem does this PR solve?

Issue Number: close #4992

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[gemini-code-assist]

Code Review

This pull request transitions checkpointTs to an atomic type within the dispatcherStat struct and refactors checkpoint update logic to use CompareAndSwap for better concurrency control. Additionally, it replaces bytes.Compare with common.StartCompare and common.EndCompare for key range checks and updates the event store iterator's boundary conditions. Feedback suggests using a monotonic increase utility for updating the dispatcher's checkpoint to prevent potential regressions from stale updates.

[gemini-code-assist]

To ensure that the dispatcher's checkpoint timestamp only moves forward and to handle potential concurrent updates safely, it is better to use util.CompareAndMonotonicIncrease instead of a direct Store. This prevents a stale update from regressing the checkpoint, which could incorrectly affect the minimum checkpoint calculation for the shared subscription.

Suggested change

	dispatcherStat.checkpointTs.Store(checkpointTs)
	util.CompareAndMonotonicIncrease(&dispatcherStat.checkpointTs, checkpointTs)