void add(FileReferenceDownload fileReferenceDownload) {
        downloads.put(fileReferenceDownload.fileReference(), fileReferenceDownload);
        downloadStatuses.put(fileReferenceDownload.fileReference());
    }
Keeps track of downloads and download status

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