'For a given room_id and stream_ordering, return the forward
extremeties of the room at that point in "time".
Throws a StoreError if we have since purged the index for
stream_orderings from that point.
Args:
room_id (str):
stream_ordering (int):
Returns:
deferred, which resolves to a list of event_ids'

def get_forward_extremeties_for_room(self, room_id, stream_ordering):

    last_change = self._events_stream_cache.get_max_pos_of_last_change(room_id)
    last_change = max(self._stream_order_on_start, last_change)
    if (last_change > self.stream_ordering_month_ago):
        stream_ordering = min(last_change, stream_ordering)
    return self._get_forward_extremeties_for_room(room_id, stream_ordering)
