void trackRequest(File file) {
        String absolutePath = file.getAbsolutePath();
        if ( ! file.exists())
            log.log(Level.WARNING, "Could not find file '" + absolutePath + "'");

        if ( ! file.setLastModified(Instant.now().toEpochMilli()))
            log.log(Level.WARNING, "Could not set last modified timestamp for '" + absolutePath + "'");
    }
Set last modification time for a file reference or downloaded url, to be able
to later clean up file references or urls not used for a long time.

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