Give temporary processes a little time to stop cleanly

Allow temporary processes to perform some final actions when shutting down.  For example, `moc_muc_room:terminate/3` fails to [send 'unavailable' presence][1] to the room participants when killed immediately.

[1]: https://github.com/processone/ejabberd/blob/cc958f7787a824f9d541a05fdbd37f4c3047ee7d/src/mod_muc_room.erl#L865