stream: clean-up packet parsing

[yjh0502]

This PR introduces parse_packet to cleanup packet parsing in Stream. The PR actually fixes several problems/bugs.

• EOF handling: AyncRead returns Ready(0) on EOF, but Stream didn't handle it correctly [1]. The PR handles the problem.
• Ensure that exact amount of data is fed into PacketParser. It seems that PacketParser required to receive exact amount of data returned by PacketParser::parse.

The PR solves problems I observed while using the library.

[1] https://github.com/blackbeam/mysql_async/blob/master/src/io/mod.rs#L298

[blackbeam]

Thanks for your work.