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

[ti-srebot]

cherry-pick #33677 to release-5.1
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/33694

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---

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]

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• 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.

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[ti-srebot]

/run-all-tests

[ti-srebot]

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

[VelocityLight]

This pull request is closed because it's related version has closed automatic cherry-picking.
If it's still needed, you can reopen it or just regenerate it using bot,
see:

• https://prow.tidb.io/command-help#cherrypick
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• https://internals.tidb.io/t/topic/785