@Override
    public void handleRequestDone(Request req) {
        Target target = (Target) req.getContext();
        if (req.isError()) {
            log.log(Level.FINE, () -> req.methodName() + " failed for " + target + ": " + req.errorCode() + " (" + req.errorMessage() + ")");
        }
        if (target != null) target.close();
    }
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