'Returns a set of room_ids the user is currently joined to'

@cachedInlineCallbacks(max_entries=500000, iterable=True)
def get_rooms_for_user(self, user_id):

    rooms = (yield self.get_rooms_for_user_where_membership_is(user_id, membership_list=[Membership.JOIN]))
    defer.returnValue(frozenset((r.room_id for r in rooms)))
