MXS-553: Add diagnostics_json to protocol API

The protocol now allows protocol modules to return JSON formatted
information about the protocol module internals. Currently this is only
implemented by the mariadbbackend module and it returns the current
connection ID on the backend server.

server/modules/protocol/MySQL/mariadbclient/mysql_client.cc

@@ -107,6 +107,7 @@ extern "C"
             NULL,                                   /* Authentication                */
             gw_default_auth,                        /* Default authenticator         */
             gw_connection_limit,                    /* Send error connection limit   */
+            NULL,
             NULL
         };