p2p: disable encryption handshake

The diff is a bit bigger than expected because the protocol handshake
logic has moved out of Peer. This is necessary because the protocol
handshake will have custom framing in the final protocol.

p2p/message.go

@@ -197,7 +197,7 @@ func (rw *frameRW) ReadMsg() (msg Msg, err error) {
 		return msg, err
 	}
 	if !bytes.HasPrefix(start, magicToken) {
-		return msg, fmt.Errorf("bad magic token %x", start[:4], magicToken)
+		return msg, fmt.Errorf("bad magic token %x", start[:4])
 	}
 	size := binary.BigEndian.Uint32(start[4:])