private CompletableFuture<Optional<File>> startDownload(FileReferenceDownload fileReferenceDownload) {
        return fileReferenceDownloader.startDownload(fileReferenceDownload);
    }
Start downloading, the future returned will be complete()d by receiving method in {@link FileReceiver}
filedistribution/src/main/java/com/yahoo/vespa/filedistribution/FileDownloader.java