scheduler(ticdc): advance checkpoint on every poll

[overvenus]

What problem does this PR solve?

Issue Number: ref #4757

What is changed and how it works?

Advance checkpoint on every poll. It helps reduce latency spike when adding a new cdc node.

Check List

Tests

• Unit test

Questions

Will it cause performance regression or break compatibility?

No

Do you need to update user documentation, design documentation or monitoring documentation?

No

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• 3AceShowHand
• sdojjy

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[overvenus]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: e457cba

[ti-chi-bot]

@overvenus: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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