double downloadStatus(FileReference fileReference) {
        double status = 0.0;
        Optional<Downloads.DownloadStatus> downloadStatus = downloadStatuses.get(fileReference);
        if (downloadStatus.isPresent()) {
            status = downloadStatus.get().progress();
        }
        return status;
    }
Keeps track of downloads and download status

@author hmusum
filedistribution/src/main/java/com/yahoo/vespa/filedistribution/Downloads.java