sink/mysql: adjust producer/consumer exit sequence to avoid goroutine leak

[amyangfei]

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.

[amyangfei]

/run-all-tests

[amyangfei]

/run-all-tests

[amyangfei]

/run-unit-tests

[amyangfei]

/run-all-tests

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• leoppro
• liuzix

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[zier-one]

/run-all-tests

[zier-one]

/merge

[ti-chi-bot]

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

Commit hash: 32673df

[codecov-commenter]

Codecov Report

Merging #1929 (49ffd92) into master (a1067a0) will increase coverage by 0.4297%.
The diff coverage is 67.3134%.

@@               Coverage Diff                @@
##             master      #1929        +/-   ##
================================================
+ Coverage   53.8503%   54.2801%   +0.4297%     
================================================
  Files           155        163         +8     
  Lines         16479      17336       +857     
================================================
+ Hits           8874       9410       +536     
- Misses         6655       6918       +263     
- Partials        950       1008        +58     

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #1945.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #1946.