code (xmpp): store version after roster item update + fix item removal: roster version is now stored after roster item has been added or removed. This way if something interrupting the process happens, former item update will be received and roster should be updated correctly. Roster item removal was crashing because `load()` is not used (local PersistentDict cache is then not updated, and the key to remove is missing there). The exception is now catched and ignored to avoid loading the data only to remove an item.