Function next_missing_event 

async fn next_missing_event<Fetch, Fut>(
    body: &Args<Request>,
    fetch: &Fetch,
    seen_max: usize,
    seen: &mut BTreeSet<OwnedEventId>,
    pending: &mut VecDeque<(OwnedEventId, bool)>,
    fetches: &mut FuturesOrdered<Fut>,
) -> Option<(OwnedEventId, CanonicalJsonObject)>
where
    Fetch: Fn((OwnedEventId, bool)) -> Fut + Sync,
    Fut: Future<Output = (OwnedEventId, bool, Result<CanonicalJsonObject>)> + Send,