*: avoid concurrently using the session in the syncload (#52830)

[ti-chi-bot]

This is an automated cherry-pick of #52830

What problem does this PR solve?

Issue Number: close #52827

Problem Summary:

What changed and how does it work?

when singleflight meets timeout, some goroutines will concurrently use reuse session. we should keep everyone using independent sessions.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

run it in the HA, It can pass test.

• No need to test

  • I checked and no code files have been changed.

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

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 5 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (release-7.5@d9788b9). Click here to learn what that means.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-7.5     #53331   +/-   ##
================================================
  Coverage               ?   71.9949%           
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  Files                  ?       1411           
  Lines                  ?     409855           
  Branches               ?          0           
================================================
  Hits                   ?     295075           
  Misses                 ?      94919           
  Partials               ?      19861           

Flag	Coverage Δ
unit	71.9949% <75.0000%> (?)

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	53.4505% <0.0000%> (?)

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-05-16 13:01:52.29315838 +0000 UTC m=+1744666.050293947: ☑️ agreed by hawkingrei.
• 2024-05-16 13:59:20.824859254 +0000 UTC m=+1748114.581994822: ☑️ agreed by winoros.