Store any incoming presence push in the local cache anyway, even if there's no interested observers (yet *hint*) (SYN-115)

Workaround for non-uniqueness of room member events in the database confusing HAVING COUNT() test of room membership intersection (with thanks to Tom Molesworth)

Squash room_ids list to a set() to remove duplicates - with TODO marker as I have no idea /why/ the dups are happening in the first place

Added the needed libssl-dev package