sink/mysql: adjust producer/consumer exit sequence to avoid goroutine leak (#1929)

[ti-chi-bot]

This is an automated cherry-pick of #1929

What problem does this PR solve?

Fix a goroutine leak bug, where the sink worker has exited, but more txns are sent to txn channel of sink worker.
And the producer will wait for the txn to be consumed forever.

What is changed and how it works?

Add a cleanup mechanism for sink worker.

1. producer(sink.flushRowChangedEvents goroutine) of txnCh exits
2. goroutine in 1 sends notification to closedCh of each sink worker
3. each sink worker receives the notification from closedCh and mark FinishWg as Done

Check List

Tests

• Unit test
• Integration test

Release note

• Fix a bug that some MySQL connection could leak after MySQL sink meets error and pauses.

[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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[amyangfei]

/run-all-tests

[amyangfei]

/run-kafka-tests

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• amyangfei
• leoppro

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[zier-one]

/merge

[ti-chi-bot]

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

Commit hash: 2f50566

[amyangfei]

/run-integration-tests

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (release-5.0@13f80b1). Click here to learn what that means.
The diff coverage is n/a.

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