scheduler: add Relay as a worker stage and fix bug (#2219)

[ti-chi-bot]

This is an automated cherry-pick of #2219

What problem does this PR solve?

close https://github.com/pingcap/dm/issues/2204

What is changed and how it works?

Now scheduler of DM master will maintain DM worker in 4 stage: Offline, Free, Relay, Bound. Relay is newly added to easily and correctly handle some stage transition.

Check List

Tests

• Unit test
• Integration test

Code changes

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

Side effects

• Increased code complexity
• Breaking backward compatibility (but fix a bug)

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation AND METRICS
• Need to be included in the release note

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• glorv
• lance6716

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[glorv]

/merge

[ti-chi-bot]

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

Commit hash: 9e01c7f62d9b96ed9a6ee33b3b210b7dd2aba929