rpcclient: Update for go1.10 breaking changes.

Go 1.10 made some changes such that json.Unmarshal can no longer
unmarshal into exported fields that are themselves embedded via an
uninitialized unexported pointer.

Since rpcclient previously relied on this behavior, this updates the
client to create the pointers before unmarshalling into the struct.

rpcclient/infrastructure.go

@@ -310,6 +310,8 @@ func (c *Client) handleMessage(msg []byte) {
 	// Attempt to unmarshal the message as either a notification or
 	// response.
 	var in inMessage
+	in.rawResponse = new(rawResponse)
+	in.rawNotification = new(rawNotification)
 	err := json.Unmarshal(msg, &in)
 	if err != nil {
 		log.Warnf("Remote server sent invalid message: %v", err)