ddl: build sessPool even if RunWorker == false (#33677)

[ti-srebot]

cherry-pick #33677 to release-4.0
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/33692

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git push git@github.com:ti-srebot/tidb.git pr/33692:release-4.0-88cd76535d57

---

Signed-off-by: wjhuang2016 huangwenjun1997@gmail.com

What problem does this PR solve?

Issue Number: close #33676

Problem Summary:
struct ddl doesn't have sessPool if RunWorker == false

What is changed and how it works?

Move the code to make sure build sessPool even if RunWorker == false

Check List

Tests

• Unit test
• Integration test
  pass integration-ddl-test
• Manual test (add detailed scripts or steps below)
• 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

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• ngaut

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[ti-chi-bot]

@ti-srebot: 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.

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[ti-srebot]

/run-all-tests

[ti-srebot]

@wjhuang2016 you're already a collaborator in bot's repo.

[xhebox]

close for DMR version