Function short_auth_chain 

async fn short_auth_chain<'a, I>(
    services: &'a Services,
    room_id: &RoomId,
    room_version: &RoomVersionId,
    heads: I,
) -> Vec<ShortEventId> 
where
    I: Iterator<Item = &'a EventId> + Clone + ExactSizeIterator + Send + 'a,