backend: fix the bug that the redirection signal may be discarded

[djshow832]

What problem does this PR solve?

Issue Number: close None

Problem Summary:
Some test cases are unstable, such as TestNormalRedirect(https://github.com/pingcap/TiProxy/runs/8181135325?check_suite_focus=true) and TestRedirectFail.

Reason:
This is a bug introduced by #72. It doesn't only happen in testing.
Since BackendConnManager.processSignals also handles mgr.redirectResCh now, this may happen:

• The process signal goroutine is executing notifyRedirectResult() and calls OnRedirectSucceed().
• The router sees that the redirection succeeds and sends another redirection signal immediately.
• The process signal goroutine is just returning from notifyRedirectResult() but does not go to the select-case yet.
• BackendConnManager.Redirect is a non-blocking function but the size of channel signalReceived is 0, it discards the signal because it needs to wait.
• The redirection signal is never sent but the router thought it had, thus the connection will never finish redirection.

What is changed and how it works:
Actually, doing one of these modifications is enough, but I modified them both to be more secure:

• Change the size of channel signalReceived to 1.
• Never discard the redirection signal.

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 (Don't forget to add the declarative for API)
• Has weirctl change
• Other user behavior changes

Release note

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None