private Future<Optional<File>> startDownload(FileReference fileReference, long sessionId, ApplicationId applicationId) {
        log.log(FINE, () -> "Downloading application package with " + fileReference +
                " for " + applicationId + " (session " + sessionId + ")");
        return fileDownloader.getFutureFileOrTimeout(new FileReferenceDownload(fileReference,
                                                                               this.getClass().getSimpleName(),
                                                                               false));
    }
Verifies that all active sessions has an application package on local disk.
If not, the package is downloaded with file distribution. This can happen e.g.
if a config server is down when the application is deployed. This maintainer should only be run
if there is more than 1 config server

@author gjoranv
configserver/src/main/java/com/yahoo/vespa/config/server/maintenance/ApplicationPackageMaintainer.java