balance, proxy: fix TiDB CPU imbalance when balance.policy="resource" (#1173) - #1175

This is an automated cherry-pick of #1173

What problem does this PR solve?

Issue Number: close #1172

Problem Summary:

• When a connection is closing and migrating concurrently, the connection may be leaked, leading to wrong CPU scores.
• Data race in ScoreBasedRoute.updateGroups().
• Other minor concurrency bugs.

What is changed and how it works:

• Add physicalOwner and scoreOwner to the backendWrapper so that it can always find the right backend.
• Remove redirectingBackendID because it's unnecessary now.
• Fix the non-atomic access of BackendConnManager.redirectResCh.
• Fix the redirect result is incorrectly set in BackendConnManager.tryRedirect.
• Other minor fixes.

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

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None