proxy: refine sqlserver.SQLServer

[xhebox]

What problem does this PR solve?

Issue Number: ref #15

Problem Summary: As title

What is changed and how it works:

1. rename to proxy.SQLServer
2. pass logger to SQLServer and ClientConnection
3. handle cleanup of connection smoothly
4. replace siddontong/go-mysql to one from go-mysql-org
5. use more errors.Collect

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

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[djshow832]

Why not use if opErr, ok := err.(*net.OpError) as TiDB does?

[xhebox]

I prefer the new style. Also, if errors are wrapped, type cast would not work. ButIs() always works without thinking much.

[xhebox]

Another thing is that there are many *net.OpError. But I think we should only return nil on ErrClosed. That is also the description of the original implementation.

[djshow832]

Also, if errors are wrapped, type cast would not work.

I think we should keep the code the same with TiDB when you can not thoroughly test it, because TiDB has been running for a long time.

That is also the description of the original implementation.

Which description? The one of TiDB or golang?

[xhebox]

I mean weir. The logging message is on closed connection and return nil.

[xhebox]

I think we should keep the code the same with TiDB when you can not thoroughly test it, because TiDB has been running for a long time.

I think it is fine. Mysql protocol did not define anything about the behavior of accept. And returnn nil if clients disconnects on its own is normal among other apps. But we should report errors if it is not ErrClosed(BTW, ErrClosed is also the only one exposed error in net package).