backend, logger: fix data race in UT

[djshow832]

What problem does this PR solve?

Issue Number: close #442

Problem Summary:

• Data race in TestLogConcurrently and TestMultiAddr
• There are some other concurrency bugs when forcibly closing the SQLServer: closing may happen anytime, including before connecting, during executing / redirecting / graceful shutdown, or concurrently with another closing

What is changed and how it works:

• Fix data race in TestLogConcurrently
• Process all the non-atomic members in BackendConnMgr with a lock to avoid data race
• Call all the interfaces in HandshakeHandler in a lock so that the interfaces are safe to read any data
• Stop the checkBackendTicker before executing commands so that long queries won't lead to unnecessary check
• Avoid overriding statusClosing with statusNotifyClose by replacing statusClose.Store with statusClose.CompareAndStore
• Fast fail when the context is canceled or the connection is closing in BackendConnMgr so that force close won't block

Note:

• NotifyBackendStatus is still unsafe. I'll refactor it in another PR Remove NotifyBackendStatus #414
• package net is always killed with -race, so I didn't add -race to go test UT in package net is always killed with -race #447

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

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[LGTM Timeline notifier]

Timeline:

• 2024-01-15 02:43:16.335155823 +0000 UTC m=+151037.899453529: ☑️ agreed by xhebox.