log-backup: implement subscription of flush

[YuJuncen]

What problem does this PR solve?

Issue Number: Ref tikv/tikv#13824

Problem Summary:
In the post, we were polling the checkpoint of each region from TiKV and calculating the global checkpoint, which isn't so real-timed.

What is changed and how it works?

This PR adapts the new interface which supports subscribing flush events.
Generally, we added a new actor named FlushSubscriber to

Check List

Tests

• Unit test
• Manual test (add detailed scripts or steps below)
  Tested via:
• TPCC workload, happy path.
• TPCC workload, restart cluster every 30 mins.
• TPCC workload, with one node using older version TiKV (which doesn't support subscribing)
• TPCC workload, upgrading from older version.

Release note

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

Improve the RPO of PITR.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Leavrth
• joccau

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[joccau]

LGTM

[YuJuncen]

/merge

[joccau]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 2ad7f64

[YuJuncen]

/run-mysql-test

[YuJuncen]

/run-unit-test

[sre-bot]

TiDB MergeCI notify

✅ Well Done! New fixed [1] after this pr merged.

CI Name	Result	Duration	Compare with Parent commit
idc-jenkins-ci/integration-cdc-test	🔴 failed 1, success 39, total 40	24 min	Existing failure
idc-jenkins-ci-tidb/integration-common-test	✅ all 17 tests passed	19 min	Fixed
idc-jenkins-ci-tidb/common-test	🟢 all 11 tests passed	10 min	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-1	🟢 all 26 tests passed	6 min 4 sec	Existing passed
idc-jenkins-ci-tidb/tics-test	🟢 all 1 tests passed	5 min 44 sec	Existing passed
idc-jenkins-ci-tidb/integration-ddl-test	🟢 all 6 tests passed	4 min 54 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-2	🟢 all 28 tests passed	4 min 24 sec	Existing passed
idc-jenkins-ci-tidb/mybatis-test	🟢 all 1 tests passed	3 min 53 sec	Existing passed
idc-jenkins-ci-tidb/integration-compatibility-test	🟢 all 1 tests passed	2 min 54 sec	Existing passed
idc-jenkins-ci-tidb/plugin-test	🟢 build success, plugin test success	4min	Existing passed