client: Support parallel TSO RPC requests on single dispatcher loop

[MyonKeminta]

What problem does this PR solve?

Issue Number: Close #8432

What is changed and how does it work?

client: Support parallel TSO RPC requests on single dispatcher loop

This commit supports handling multiple TSO RPC concurrently in one single dispatcher loop to reduce the expected time that each GetTS call spent on waiting the next batch.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has the configuration change
• Has HTTP API interfaces changed (Don't forget to add the declarative for the new API)
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tiup:
• Need to cherry-pick to the release branch

Release note

None.

[ti-chi-bot]

Skipping CI for Draft Pull Request.
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You can still manually trigger a test run with /test all

[ti-chi-bot]

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