operator: pause scheduler after all connections established (#51823)

[ti-chi-bot]

This is an automated cherry-pick of #51823

What problem does this PR solve?

Issue Number: ref #51448

Problem Summary:

In #51449, we delayed pausing admin commands after all connections are established. But that isn't enough, due to PD schedulers are paused and ID allocation is disabled.

What changed and how does it work?

This PR pauses PD's scheduler after all connections are established.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

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[ti-chi-bot]

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[ti-chi-bot]

@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/build	02e0781	link	true	/test build
idc-jenkins-ci-tidb/unit-test	02e0781	link	true	/test unit-test
idc-jenkins-ci-tidb/check_dev	02e0781	link	true	/test check-dev
idc-jenkins-ci-tidb/check_dev_2	02e0781	link	true	/test check-dev2

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[YuJuncen]

should be picked in #53027