log_backup: stop task while memory out of quota (#16008)

[ti-chi-bot]

This is an automated cherry-pick of #16008

What is changed and how it works?

Issue Number: Close #15414

What's Changed:

This PR have refactored the subscription manager.
Generally, this:
- Replace the instance itself with a handle. This make it a real reactor(with an real event loop).
- Handle the result of subscripting a region via the message system instead of asynchronously, this will be the basis of making subscription tracker thread safe and (someday, hopefully) merge the basic libraries with TiCDC.

Based on the changes above, this PR also allows a region to be temporarily deregistered while we are about to reach the memory quota.

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch

Check List

Tests

• Integration test
• Manual test (add detailed scripts or steps below)

Release note

Reduced the possibility of run out of memory when there are huge transactions.

[ti-chi-bot]

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[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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