MXS-1847: Make server parameter updates atomic

The updates to server parameters are now performed in an atomic manner.

include/maxscale/server.h

@@ -311,6 +311,15 @@ void server_add_parameter(SERVER *server, const char *name, const char *value);
  */
 bool server_remove_parameter(SERVER *server, const char *name);
 
+/**
+ * @brief Update server parameter
+ *
+ * @param server Server to update
+ * @param name   Parameter to update
+ * @param value  New value of parameter
+ */
+void server_update_parameter(SERVER *server, const char *name, const char *value);
+
 /**
  * @brief Check if a server points to a local MaxScale service
  *