ttl: reschedule task to other instances when shrinking worker (#57703)

[ti-chi-bot]

This is an automated cherry-pick of #57703

What problem does this PR solve?

Issue Number: close #57990

What changed and how does it work?

This PR makes some changes:

1. Flush expired rows not deleted in delWorker before it stops.
2. When stopping a scanWorker, wait for all rows to be deleted before writing its state to task meta.
3. Mark the status of a task back to waiting when its worker stops to ensure it can be obtained by other instances again.
4. When task heartbeating, merge the current statistics and the old one that recovered from the task meta.
5. Add more logs.

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]

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@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/unit-test	0abfcc4	link	true	/test unit-test
idc-jenkins-ci-tidb/build	0abfcc4	link	true	/test build
idc-jenkins-ci-tidb/check_dev	0abfcc4	link	true	/test check-dev
idc-jenkins-ci-tidb/check_dev_2	0abfcc4	link	true	/test check-dev2
idc-jenkins-ci-tidb/mysql-test	0abfcc4	link	true	/test mysql-test

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