implement proper protocol versioning

[babymotte]

Client and server must negotiate a protocol version to use during the handshake. The Client send a handshake request on connection giving a list a protocol versions it supports.
The server picks the latest protocol version it supports and includes it in its handshake response. If the server does not support any of the requested protocol versions it must send an error instead of a handshake response.
Messages exchanged between the server and client must then all conform to the negotiated protocol version.