Protocol refs: https://github.com/NetEase/pomelo/wiki/Communication-Protocol -<type>-|--------<length>--------|-<data>- --------|------------------------|-------- 1 byte packet type, 3 bytes packet data length(big end), and data segment.

Unpack binary data to packet, if packet has not been received completely, return nil and incomplete data, concrete protocol ref pack function.

data packet.

packet for handshake request(client) <====> handshake response(server).

packet for handshake ack from client to server.

heartbeat packet.

disconnect message from server.