MXS-2559: Log source of loaded users

MySQLAuth now logs the server where the users were loaded from. As only
the initial loading of users causes a log message, it is still possible
for the source server to change without any indication of it.

server/modules/authenticator/MySQLAuth/mysql_auth.h

@@ -198,10 +198,11 @@ bool dbusers_save(sqlite3* src, const char* filename);
  *
  * @param service    The current service
  * @param skip_local Skip loading of users on local MaxScale services
+ * @param srv        Server where the users were loaded from (output)
  *
  * @return -1 on any error or the number of users inserted (0 means no users at all)
  */
-int replace_mysql_users(SERV_LISTENER* listener, bool skip_local);
+int replace_mysql_users(SERV_LISTENER* listener, bool skip_local, SERVER** srv);
 
 /**
  * @brief Verify the user has access to the database