Avoid recoverable panic before handshake stage

[YangKeao]

Development Task

Now TiProxy might report recoverable panic in many protocol related codes:

1. For the packets before handshake.
2. For the packets after handshake.
3. For the packets from TiDB to client.

For this issue, I'm only going to fix the first class. Because: we can assume the TiDB always gives the correct format of packets (or there might be some incompability), and we might assume the client after logging in will always give a correct protocol.

For now, usually the recoverable panic is caused by the port scanning software on the internet.