*: fix scheduling can not immediately start after transfer leader (#4875)

[ti-chi-bot]

This is an automated cherry-pick of #4875

Signed-off-by: Ryan Leung [email protected]

What problem does this PR solve?

Issue Number: Close #4769.

What is changed and how does it work?

This PR uses the flag to distinguish if the region's heartbeat is new when transferring the PD leader.

Check List

Tests

• Unit test

Release note

Fix the issue that scheduling cannot immediately start after PD leader transfers

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has not been approved.

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Reviewer can indicate their review by submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[ti-chi-bot]

@ti-chi-bot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

To merge this cherry pick, it must first be approved by the collaborators.

AFTER it has been approved by collaborators, please ping the release team in a comment to request a cherry pick review.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.