schedulers, handler: fix the evict-leader-scheduler sync

[JmPotato]

What problem does this PR solve?

Issue Number: ref #5839.

What is changed and how does it work?

- Pass the correct `removeSchedulerCb` when the server is in API mode.
- Pause and resume the store leader transfer when reloading the cfg.

Check List

Tests

• Unit test
• Integration test

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• lhy1024
• rleungx

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[JmPotato]

/merge

[ti-chi-bot]

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[ti-chi-bot]

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

Commit hash: f6831f3

[ti-chi-bot]

@JmPotato: Your PR was out of date, I have automatically updated it for you.

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