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]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• amyangfei

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[amyangfei]

/merge

[ti-chi-bot]

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

Commit hash: 75495e6

[ti-chi-bot]

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[codecov-commenter]

Codecov Report

Merging #1945 (5286277) into release-4.0 (753073a) will decrease coverage by 0.0143%.
The diff coverage is 62.2777%.

@@                 Coverage Diff                 @@
##           release-4.0      #1945        +/-   ##
===================================================
- Coverage      53.7661%   53.7518%   -0.0144%     
===================================================
  Files              153        163        +10     
  Lines            15958      17365      +1407     
===================================================
+ Hits              8580       9334       +754     
- Misses            6475       7046       +571     
- Partials           903        985        +82