router: fix connection score becomes wrong when a connection closes during redirecting

[djshow832]

What problem does this PR solve?

Issue Number: close #448

Problem Summary:
There were 2 ways to update the connection score correctly when a connection closes after receiving a redirection signal:

• The connection passes the target backend to the router when it closes
• The router requests for the target backend when the connection closes

However, both these 2 ways are removed, and the connection score is incorrectly maintained.

What is changed and how it works:
Maintain the target backend in the router itself. When a connection is closed during redirecting, get the target backend from the router itself.

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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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-01-16 05:32:06.842628199 +0000 UTC m=+247568.406925904: ☑️ agreed by xhebox.