unit(dm): add Kill func for unit (#4035)

[ti-chi-bot]

This is an automated cherry-pick of #4035

What problem does this PR solve?

Issue Number: close #3737

Root case of #3737

• when dm-master node offline, dm-worker will lost the keepalive with master
• dm-worker will use stopWorker to stop running tasks
• if current running unit is syncer, syncer will try continue syncing until current trasaction end

  tiflow/dm/syncer/syncer.go

  Line 1411 in abefe57

  tctx.L().Info("received subtask's done")

• at the same time master online again or another master node beacme leader will try scheduler task to another running worker
• so there are two workers syncning from same source ,and we may meet duplicate key error

What is changed and how it works?

• kill running subtasks when worker lost the keep-alive with dm-master (both for master offline or worker offline )

• add Kill interface func to unit

• impleament kill in syncer unit

• change worker.Close to worker.Stop(graceful bool), when ungraceful stop, worker will call killAllSubTasks to force stop and this will cancel syncer.Run immediately

• only when worker lost keepalive unit should call Kill

Check List

Tests

• Unit test
• Integration test

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change

Side effects

• Possible performance regression
• Increased code complexity

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation
• Need to update key monitor metrics in both TiCDC document and official document

Release note

`fix a bug that multiple worker write for same upstream`.

[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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[Ehco1996]

this cp is rejected closed now