relay(dm): refactor handleFileHoleExist for relay writer

[zhangjinpeng87]

What problem does this PR solve?

Issue Number: close #9563

What is changed and how it works?

• Refactor handleFileHoleExist, make it only focus on handling File Hole.
• Add a private detectDuplicateEvent function to detect duplicate event

Check List

Tests

• Unit test

Questions

Will it cause performance regression or break compatibility?

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

Release note

None

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[ti-chi-bot]

@zhangjinpeng1987: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
jenkins-ticdc/verify	42fe808	link	true	/test verify
pull-dm-integration-test	42fe808	link	true	/test dm-integration-test
pull-engine-integration-test	42fe808	link	true	/test engine-integration-test
pull-dm-compatibility-test	42fe808	link	true	/test dm-compatibility-test

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