worker: enable relay and handling subtask on demand (#1508)

[ti-srebot]

cherry-pick #1508 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/1527

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

git push git@github.com:ti-srebot/dm.git pr/1527:release-2.0-33c2a59befbb

---

What problem does this PR solve?

call EnableRelay when source config enables relay, and call EnableHandleSubTask when receive a source bound. In future we will call EnableRelay by watching another prefix in etcd

What is changed and how it works?

Check List

Tests

• Unit test

Code changes

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

Side effects

• Possible performance regression

Related changes

• Need to cherry-pick to the release branch

[lance6716]

/run-all-tests

[lance6716]

/run-compatibility-tests

[lance6716]

/lgtm

[GMHDBJD]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• GMHDBJD
• lance6716

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[lance6716]

/merge

[ti-chi-bot]

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

Commit hash: 97a547e