mcs: add lock for forward tso stream (#9095)

[ti-chi-bot]

This is an automated cherry-pick of #9095

What problem does this PR solve?

Issue Number: Close #9091

What is changed and how does it work?

Check List

Tests

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

go test -benchmem -run=^$ -bench ^BenchmarkForwardTsoConcurrently$ github.com/tikv/pd/tests/integrations/mcs/tso -benchtime 120s

client = 3
164         776899433 ns/op        52592338 B/op     446288 allocs/op
182         779831256 ns/op        52671058 B/op     446771 allocs/op
176         776116538 ns/op        52677991 B/op     446741 allocs/op

client = 1
386         342842281 ns/op        17493004 B/op     146447 allocs/op
390         344128967 ns/op        17487349 B/op     146318 allocs/op
379         356701493 ns/op        17528155 B/op     146427 allocs/op

Release note

None.

[codecov]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 75.61%. Comparing base (727c208) to head (212b7cc).
Report is 1 commits behind head on release-8.5.

Additional details and impacted files

@@               Coverage Diff               @@
##           release-8.5    #9106      +/-   ##
===============================================
- Coverage        75.63%   75.61%   -0.03%     
===============================================
  Files              462      462              
  Lines            72379    72389      +10     
===============================================
- Hits             54744    54736       -8     
- Misses           14123    14155      +32     
+ Partials          3512     3498      -14     

Flag	Coverage Δ
unittests	75.61% <93.33%> (-0.03%)	⬇️

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nolouch, rleungx

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-02-27 09:03:25.642777458 +0000 UTC m=+519353.595935723: ☑️ agreed by rleungx.
• 2025-02-28 02:15:05.068748124 +0000 UTC m=+581253.021906382: ☑️ agreed by nolouch.