net: complete some definitions

[xhebox]

What problem does this PR solve?

Issue Number: ref #248

Problem Summary: The current backend must convert from *PacketIO oriented to []byte/MysqlPacket oriented. Command processor or authenticator should not read from the connection actively. To do this, we should provide a set of helpers that deals with packets, instead of *PacketIO.

What is changed and how it works:

1. This packet completes mysql.ComXXX definition. The next one will provide more helpers.
2. fix post merge CI.

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