public void close() {
        executor.ifPresent(ex -> {
            ex.shutdownNow();
            try {
                if (! ex.awaitTermination(10, TimeUnit.SECONDS))
                    throw new RuntimeException("Unable to shutdown " + executor + " before timeout");
            } catch (InterruptedException e) {
                throw new RuntimeException(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