Only serialize created servers

Any changes to servers that aren't created online need to be manually
added to the configuration file in order for them to take effect after the
restart.

include/maxscale/server.h

@@ -117,6 +117,7 @@ typedef struct server
     int            persistmax;     /**< Maximum pool size actually achieved since startup */
     uint8_t        charset;        /**< Default server character set */
     bool           is_active;      /**< Server is active and has not been "destroyed" */
+    bool           created_online; /**< Whether this server was created after startup */
 #if defined(SS_DEBUG)
     skygw_chk_t    server_chk_tail;
 #endif