*: add sql variable @@tidb_accelerate_user_creation_update

[tiancaiamao]

What problem does this PR solve?

Issue Number: ref #55563

Problem Summary:

What changed and how does it work?

Add a switch to control the behavior.
If @@tidb_accelerate_user_creation_update = off, use the old behavior, i.e. load all the user data
If @@tidb_accelerate_user_creation_update = on, only load the active user

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

• 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

[tiprow]

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[codecov]

Codecov Report

Attention: Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.

Project coverage is 74.7787%. Comparing base (7515439) to head (66a0ebb).
Report is 4 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #58512        +/-   ##
================================================
+ Coverage   73.0495%   74.7787%   +1.7291%     
================================================
  Files          1690       1736        +46     
  Lines        467261     476082      +8821     
================================================
+ Hits         341332     356008     +14676     
+ Misses       104958      97597      -7361     
- Partials      20971      22477      +1506     

Flag	Coverage Δ
integration	48.7874% <88.8888%> (?)
unit	72.2366% <94.4444%> (-0.0019%)	⬇️

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Components	Coverage Δ
dumpling	52.6910% <ø> (ø)
parser	∅ <ø> (∅)
br	60.5565% <ø> (+15.0563%)	⬆️

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Copilot reviewed 5 out of 7 changed files in this pull request and generated no comments.

Files not reviewed (2)

• pkg/privilege/privileges/BUILD.bazel: Language not supported
• pkg/privilege/privileges/tidb_auth_token_test.go: Evaluated as low risk

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-02-12 01:43:58.055329961 +0000 UTC m=+407280.451552023: ☑️ agreed by lcwangchao.
• 2025-02-12 01:54:04.797384806 +0000 UTC m=+407887.193606867: ☑️ agreed by lance6716.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Frank945946, lance6716, lcwangchao, yudongusa

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• OWNERS [lance6716,lcwangchao]
• pkg/domain/OWNERS [lance6716]
• pkg/sessionctx/vardef/OWNERS [yudongusa]
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