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.

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-02-27 09:01:56.768192495 +0000 UTC m=+519264.721350753: ☑️ agreed by rleungx.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rleungx

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 [rleungx]

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