server: make clientConn() thread-safe (#49073)

[ti-chi-bot]

This is an automated cherry-pick of #49073

What problem does this PR solve?

Issue Number: ref #48224, #36793 and #32110

Problem Summary:

What changed and how does it work?

Please check #36793 (comment) for more details and context:

But for further enhancement, we cloud still try to make closeConn thread-safe.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• None

Documentation

• None

Release note

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

None

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-7.5@63e621a). Click here to learn what that means.
The diff coverage is 76.4705%.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-7.5     #50107   +/-   ##
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  Coverage               ?   71.9650%           
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  Files                  ?       1403           
  Lines                  ?     407234           
  Branches               ?          0           
================================================
  Hits                   ?     293066           
  Misses                 ?      94434           
  Partials               ?      19734           

Flag	Coverage Δ
unit	71.9650% <76.4705%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	53.9913% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	53.3046% <0.0000%> (?)

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hawkingrei, tiancaiamao

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-02-07 08:01:55.18300293 +0000 UTC m=+343840.749772804: ☑️ agreed by hawkingrei.
• 2024-02-19 08:40:36.858162365 +0000 UTC m=+260125.605785461: ☑️ agreed by tiancaiamao.