scheduler: close workers grpc connections timely (#1639)

[ti-srebot]

cherry-pick #1639 to release-2.0
You can switch your code base to this Pull Request by using git-extras:

# In dm repo:
git pr https://github.com/pingcap/dm/pull/1643

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/dm.git pr/1643:release-2.0-c88ea874ef83

---

What problem does this PR solve?

part 2 of #1636
dm-master didn't close grpc connections before it close scheduler.

What is changed and how it works?

Close scheduler.workers grpc connections when scheduler fails to start or trying to Close.

Check List

Tests

• Unit test
• Integration test

Code changes

• Has exported function/method change

Related changes

• Need to cherry-pick to the release branch
• Need to be included in the release note

[lichunzhu]

/label require-LGT-1

[ti-chi-bot]

@lichunzhu: The label(s) require-lgt-1 cannot be applied. These labels are supported: Hacktoberfest, TiDB-5.0, already-cherry-pick-1.0, already-cherry-pick-2.0, already-update-dm-ansible, already-update-docs, already-update-release-note, dependencies, good first issue, help wanted, high-performance, hptc, javascript, needs-cherry-pick-1.0, needs-cherry-pick-2.0, needs-update-dm-ansible, needs-update-docs, needs-update-release-note, question, release-track.

In response to this:

/label require-LGT-1

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[lichunzhu]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• lichunzhu

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[lichunzhu]

/merge

[ti-chi-bot]

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

Commit hash: 136784e