Function download_multi 

pub async fn download_multi<R, W>(
    puller: R,
    pusher: W,
    options: DownloadOptions,
) -> DownloadResult<TokioExecutor<R, W::Error>, R::Error, W::Error>
where
    R: RandPuller + 'static + Sync,
    W: RandPusher + 'static,