router, backend: check target health before redirection

[djshow832]

What problem does this PR solve?

Issue Number: close #408

Problem Summary:

• When the connection begins redirection, the target backend may become unhealthy. This will introduce unnecessary errors.
• In ensureBackend, the newly added backend may be removed by adjustBackendList.

What is changed and how it works:

• Pass BackendInst interface (instead of addr) to the BackendConnMgr in Redirect()
• Check the health by BackendInst.Healthy() before redirection. If it's unhealthy, abort redirection
• Add locks to backendWrapper because status may be read and written concurrently
• Add a parameter removeEmpty to adjustBackendList and pass false to it in ensureBackend

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:

• 2023-12-15 02:18:57.340440743 +0000 UTC m=+581828.377667666: ☑️ agreed by xhebox.