br: use one shot session to close domain ASAP (#36558)

[ti-srebot]

cherry-pick #36558 to release-6.2
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/36594

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tidb.git pr/36594:release-6.2-6ae88c4309e1

---

What problem does this PR solve?

Issue Number: ref #36546

Problem Summary:
when we use createSession, the side effect is it will create a domain internally by kv.store. even if we drop the session the domain still exists during the whole backup. and when the domain exists, it will put an empty ip address node to etcd. this will cause the in-memory cop request slow down.

What is changed and how it works?

I add a function to close domain when finished getGlobalVariable/getDDLJobs during backup to reduce the time of hold domain/session during backup.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  before this PR, when BR getting in stuck. we can see two server info keys.

with this PR, when BR getting in stuck. we can see only one server info key and in log we can check the log "one shot domain closed"

• No code

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]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• YuJuncen
• fengou1

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[ti-srebot]

/run-all-tests

[ti-srebot]

@3pointer you're already a collaborator in bot's repo.

[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/5ba7e959ec82ac3df2b650634c8705dd235ccfe2

[3pointer]

/merge

[ti-chi-bot]

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

Commit hash: 1481560

[ti-chi-bot]

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