scheduler: replace pauseLeader with two flags and add source filter to transferIn (#8623)

[ti-chi-bot]

This is an automated cherry-pick of #8623

What problem does this PR solve?

Issue Number: Close #8621

What is changed and how does it work?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  Before this pr:
  
  After this pr:
  

Release note

None.

[ti-chi-bot]

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[ti-chi-bot]

@okJiang This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[ti-chi-bot]

@ti-chi-bot: ## If you want to know how to resolve it, please read the guide in TiDB Dev Guide.

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[ti-chi-bot]

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

Test name	Commit	Details	Required	Rerun command
pull-build-release-7.5	0d92c85	link	true	/test pull-build-release-7.5

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