Added flushing of all logs to disk when MaxScale exits

It is possible that messages logged immediately before exiting are not flushed
to disk. Flushing all logs before exiting from the main function guarantees
that any relevant log messages are flushed to disk.

server/core/gateway.c

@@ -2007,6 +2007,8 @@ int main(int argc, char **argv)
 
 return_main:
 
+    mxs_log_flush_sync();
+
     if (daemon_mode && rc != MAXSCALE_SHUTDOWN)
     {
         /** Notify the parent process that an error has occurred */