kv(ticdc): fix data loss when upstream txn conflicts during scan (#5477)

[ti-chi-bot]

This is an automated cherry-pick of #5477

What problem does this PR solve?

Issue Number: close #5468

What is changed and how it works?

Do not match prewrite and commit util it receives init event.
See more in #5468

Check List

Tests

• Unit test

Will it cause performance regression or break compatibility?

It caches more events in memory during incremental scan, it cause OOM when upstream workload is write-heavy.

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

No.

Release note

Fix data loss when upstream transaction conflicts during cdc reconnection 

[ti-chi-bot]

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[ti-chi-bot]

@ti-chi-bot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

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[VelocityLight]

This pull request is closed because it's related version has closed automatic cherry-picking.
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• https://internals.tidb.io/t/topic/785