Enhance roster to be able to hold non-persistent roster items

By default, Roster will try to persist roster items to the database. Add a way to specify specify which roster items are persistent or not. Roster items that are not persistent could be used as a way to implement dynamic roster items (roster items that are added on the fly and only in memory). This feature combined with https://igniterealtime.atlassian.net/browse/JM-807#icft=JM-807 is all we need to let modules/plugins/components handle dynamic roster items.