'Returns the set of all hosts currently in the room'

@cachedInlineCallbacks(max_entries=100000, iterable=True, cache_context=True)
def get_hosts_in_room(self, room_id, cache_context):

    user_ids = (yield self.get_users_in_room(room_id, on_invalidate=cache_context.invalidate))
    hosts = frozenset((get_domain_from_id(user_id) for user_id in user_ids))
    defer.returnValue(hosts)
