Sort child events according to MSC1772 for the spaces summary API. (#9954)

This should help ensure that equivalent results are achieved between
homeservers querying for the summary of a space.

This implements modified MSC1772 rules, according to MSC2946.
The different is that the origin_server_ts of the m.room.create event
is not used as a tie-breaker since this might not be known if the
homeserver is not part of the room.