private static Instant fileLastModifiedTime(Path fileReference) {
        try {
            BasicFileAttributes fileAttributes = readAttributes(fileReference, BasicFileAttributes.class);
            return fileAttributes.lastModifiedTime().toInstant();
        } catch (IOException e) {
            throw new UncheckedIOException(e);
        }
    }
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