@Override
    public void run() {
        if (executor.isEmpty()) return;

        try {
            deleteUnusedFiles(fileReferencesDownloadDir);
            deleteUnusedFiles(urlDownloadDir);
        } catch (Throwable t) {
            log.log(Level.WARNING, "Deleting unused files failed. ", t);
        }
    }
Deletes file references and url downloads that have not been used for some time.
See {@link com.yahoo.vespa.config.proxy.filedistribution.RequestTracker} for how we track
when a file reference or download was last used.

@author hmusum
config-proxy/src/main/java/com/yahoo/vespa/config/proxy/filedistribution/FileReferencesAndDownloadsMaintainer.java