SSL SNI routing

[xhebox]

• Without storing username and password, proxy can not complete the authentication without real backend servers. Proxy needs to fake handshake and response for both clients and servers. But if we route by SNI, we only need to fake inital handshake for clients and parse SSL request from clients. It greatly simplify the parsing of mysql protocols, leaving clients to real servers.
• This can remove literally lots of code of the complex frontend/backend logic.
  • proxy: remove domain types #16 preparation, merged into proxy: remove driver and queryctx #18
  • proxy: remove driver and queryctx #18 preparation step two
  • proxy: refine sqlserver.SQLServer #27