backend, net: close the client connection when the backend is down

[djshow832]

What problem does this PR solve?

Issue Number: close #195

Problem Summary:
On the serverless tier, when the client is idle for 5 minutes, the backend will quit. But the TiProxy doesn't know the backend quits because it's still reading the client. Then, the serverless tier thinks the connection is still alive and then wakes up a new TiDB instance.

So, TiProxy should always detect the backend and disconnect the client when the backend quits.

What is changed and how it works:

• Add a ticker on BackendConnMgr to check the backend periodically in the background goroutine
• Wrap the PacketIO.conn with a buffer and add PacketIO.IsPeerActive()
• Update the parameters of NewBackendConnMgr to make them configurable in the tests
• Fix checkConnClosed4Proxy. It never worked before because ts.mp.err is not checked

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

1. Start a MySQL client to connect to TiProxy
2. Kill the TiDB
3. The TiProxy shows that the client connection is closed within 1 minute

[2023/01/18 19:17:37.985 +08:00] [INFO] [main.nsmgr.router] [router/router_score.go:302] [update backend] [namespace=default] [addr=127.0.0.1:4000] [prev_status=healthy] [cur_status=down]
[2023/01/18 19:18:20.330 +08:00] [INFO] [main.proxy.conn.be] [backend/backend_conn_mgr.go:532] [backend connection is closed, close client connection] [connID=0] [remoteAddr=127.0.0.1:57965] [client=127.0.0.1:57965] [backend=127.0.0.1:4000]
[2023/01/18 19:18:20.331 +08:00] [INFO] [main.proxy] [proxy/proxy.go:166] [connection closed] [connID=0] [remoteAddr=127.0.0.1:57965]

Notable changes

• Has configuration change
• Has HTTP API interfaces change (Don't forget to add the declarative for API)
• Has tiproxyctl change
• Other user behavior changes

Release note

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None