Function collect_left_rooms 

fn collect_left_rooms<'a>(
    services: &'a Services,
    sender_user: &'a UserId,
    since: u64,
    next_batch: u64,
    full_state: bool,
    state_after: StateAfter,
    filter: &'a FilterDefinition,
) -> impl Future<Output = BTreeMap<OwnedRoomId, LeftRoom>> + Send + 'a