private static int outDatedFilesToKeep() {
        String env = System.getenv("VESPA_KEEP_FILE_REFERENCES_COUNT");
        if (env != null && !env.isEmpty())
            return Integer.parseInt(env);
        else
            return defaultOutdatedFilesToKeep;
    }
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