server: Set default drainClientWait as 15s (#42134) (#48944)

[crazycs520]

This is an automated cherry-pick of #42134

What problem does this PR solve?

Issue Number: close #32110

Problem Summary:

What is changed and how it works?

ref #32111

Set default drainClientWait as 15s

The default keeps waiting maybe too rigorous, It is easy having little slow txns which we may not want to wait for them, also the idle connections may block the shutdown two.
generally, 15s is enough for an ongoing txn to finish, better provider a dynamic way to set this value.

Check List

Tests

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

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

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