Allow to set a number of days for empty rooms to show up in the list (multiple conference services)

According to system properties list and this forum thread, the system property xmpp.muc.unload.empty_days has stopped working after multiple conference services feature was introduced in 3.6.0 version. This property allowed to set a number of days before a room is unloaded from a memory and from a discovery list after being empty (no visitors). Not sure if this was intended, or it was broken by the changes in conference services handling. Probably it could be done for this property to work with current Openfire version. Either a separate setting for every service or a single for all. Maybe it can be done into a GUI option for every service.