mcs: fix potential data race in scheduling server

[lhy1024]

What problem does this PR solve?

Issue Number: Close #8538

What is changed and how does it work?

Check List

Tests

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

for i in {1..100}; do
	go clean -testcache
	go test -timeout 120s -run ^TestAPI$ github.com/tikv/pd/tests/integrations/mcs/scheduling -race
done

Release note

None.

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 77.54%. Comparing base (6bf980d) to head (795ac6b).
Report is 39 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8539      +/-   ##
==========================================
- Coverage   77.55%   77.54%   -0.01%     
==========================================
  Files         474      474              
  Lines       61861    61859       -2     
==========================================
- Hits        47974    47969       -5     
- Misses      10355    10361       +6     
+ Partials     3532     3529       -3     

Flag	Coverage Δ
unittests	77.54% <25.00%> (-0.01%)	⬇️

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

[HuSharp]

Do we need to add a break label to indicate jumping out of the “for loop”?

[lhy1024]

I think it is not necessary.

[okJiang]

ref #8538 (comment)

Is there a way to make s.GetClient().XXXX() non-suspending or report timeout errors when campaigning primary?

[lhy1024]

ref #8538 (comment)

Is there a way to make s.GetClient().XXXX() non-suspending or report timeout errors when campaigning primary?

There seems to be no easy way to implement this, and similar problems may also occur in scenarios where there are network problems.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: HuSharp, okJiang

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [HuSharp]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-08-16 07:22:28.577935105 +0000 UTC m=+511833.281404743: ☑️ agreed by HuSharp.
• 2024-08-21 03:47:23.758740397 +0000 UTC m=+323638.893190518: ☑️ agreed by okJiang.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #8641.