router, backend: show server version from TiDB

[djshow832]

What problem does this PR solve?

Issue Number: close #272, #217

Problem Summary:

• When a client connects to the dedicated tier, the server version is always 5.7.25-TiDB-None
• We can't see why TiProxy thinks a TiDB is down when troubleshooting

What is changed and how it works:

• Replace BackendStatus with backendHealth to store serverVersion and pingErr
• Read the server version from the backend during health checks
• Send the server version of any backend to the client when handshake. It may be wrong when there are multiple versions running
• Log the error why TiProxy can't connect to TiDB when TiDB is removed from the router

Check List

Tests

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

1. Start a TiDB cluster with v7.0.0
2. Start a MySQL client to connect to it and the client shows Server version: 5.7.25-TiDB-v7.0.0 TiDB Server (Apache License 2.0) Community Edition, MySQL 5.7 compatible
3. Scale in one TiDB and TiProxy logs [update backend] [namespace=default] [backend_addr=127.0.0.1:4000] [prev="status: healthy"] [cur="status: down, err: http status 500: connect status port failed"]

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