MXS-862: Refactor backend authentication handling

The backend responses are now read in one place and the functions just
read the data. The protocol level will now handle the packet gathering
process and the authentication part just inspects the data.

Backend connections now load authenticators when they are being
connected. In the future, this enables the use of authentication modules
for backend connection.

server/core/server.c

@@ -83,6 +83,7 @@ server_alloc(char *servname, char *protocol, unsigned short port)
 #endif
     server->name = servname;
     server->protocol = protocol;
+    server->authenticator = NULL;
     server->port = port;
     server->status = SERVER_RUNNING;
     server->node_id = -1;