@Override
    public void triggerDownload(String hostName, int port, Set<FileReference> fileReferences) {
        Target target = supervisor.connect(new Spec(hostName, port));
        Request request = new Request("filedistribution.triggerDownload");
        request.setContext(target);
        request.parameters().add(new StringArray(fileReferences.stream().map(FileReference::value).toArray(String[]::new)));
        request.parameters().add(new StringValue(new Spec(com.yahoo.net.HostName.getLocalhost(), 19070).toString()));
        log.log(Level.FINE, () -> "Executing " + request.methodName() + " against " + target + ": " + fileReferences);
        target.invokeAsync(request, rpcTimeout, this);
    }
Notifies client which file references it should start downloading. It's OK if the call does not succeed,
as this is just a hint to the client to start downloading. Currently the only client is the config server

@param hostName       host which should be notified about file references to download
@param port           port which should be used when notifying
@param fileReferences set of file references to start downloading
configserver/src/main/java/com/yahoo/vespa/config/server/filedistribution/FileDistributionImpl.java