drainer: fix placement policy compatibility

[lichunzhu]

What problem does this PR solve?

Issue Number: close #1118 and close pingcap/tidb#31611

What is changed and how it works?

make drainer compatible with placement policy

Check List

Tests

• Unit test
• Integration test

Related changes

• Need to cherry-pick to the release branch

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• glorv
• lance6716

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[lichunzhu]

/run-all-tests

[lichunzhu]

/run-all-tests

[lichunzhu]

/run-unit-test

[lichunzhu]

/run-all-tests

[lichunzhu]

/cc glorv

[lichunzhu]

/merge

[ti-chi-bot]

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

Commit hash: d80dd6d

[ti-chi-bot]

In response to a cherrypick label: cannot checkout 5.4: error checking out 5.4: exit status 1. output: error: pathspec '5.4' did not match any file(s) known to git

[lichunzhu]

/cherry-pick release-5.4

[ti-chi-bot]

@lichunzhu: new pull request created: #1120.

In response to this:

/cherry-pick release-5.4

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