directory: update do its work inside a thread

A lot of the preparation was needed (and done in previous
months) in making update thread-safe, but here it is.

This was the first thing I made work inside a thread when I
started mpd-uclinux many years ago, and also the last thing I've
done in mainline mpd to work inside a thread, go figure.

src/main.c

@@ -444,7 +444,7 @@ int main(int argc, char *argv[])
 	       COMMAND_RETURN_KILL != handlePendingSignals()) {
 		syncPlayerAndPlaylist();
 		client_manager_expire();
-		readDirectoryDBIfUpdateIsFinished();
+		reap_update_task();
 	}
 
 	write_state_file();