Serialize server weighting parameters

The server weighting parameters couldn't be altered online and they
weren't serialized to disk. Only servers that are created online will have
their weighting parameters persisted to disk.

server/core/maxscale/service.h

@@ -109,6 +109,9 @@ int      service_isvalid(SERVICE *service);
  */
 bool service_server_in_use(const SERVER *server);
 
+/** Update the server weights used by services */
+void service_update_weights();
+
 /**
  * Alteration of the service configuration
  */