void put(FileReference fileReference, double progress) {
            downloadStatus.put(fileReference, new DownloadStatus(fileReference, progress));
            if (downloadStatus.size() > maxEntries) {
                Map.Entry<FileReference, DownloadStatus> oldest =
                        Collections.min(downloadStatus.entrySet(), Comparator.comparing(e -> e.getValue().created));
                downloadStatus.remove(oldest.getKey());
            }
        }
Keeps track of downloads and download status

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