proxy, config: add config graceful-close-conn-timeout

[djshow832]

What problem does this PR solve?

Issue Number: close #399

Problem Summary:
On the cloud, NLB takes around 90s to mark TiProxy offline after TiProxy shuts down the HTTP port. During this process, new connections are denied.

What is changed and how it works:

• Only reject new connections after graceful-wait-before-shutdown, not before
• Add a configuration graceful-close-conn-timeout. After graceful-wait-before-shutdown, reject new connections and close connections that have finished transactions, until graceful-close-conn-timeout ends

Check List

Tests

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

Notable changes

• Has configuration change
• Has HTTP API interfaces change
• Has tiproxyctl change
• Other user behavior changes

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xhebox

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• OWNERS [xhebox]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-11-16 02:57:03.031168071 +0000 UTC m=+4304220.618278200: ☑️ agreed by xhebox.

[djshow832]

/retest