Separate health check service port for proxy-server

[ameba23]

This is a feature from cvm-reverse-proxy which should be ported here.

func startHealthCheckServer(listenAddr string) {
	log.With("healthCheckListenAddr", listenAddr).Info("Starting health check server")
	healthCheckServer := &http.Server{
		Addr: listenAddr,
		Handler: http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			w.WriteHeader(http.StatusOK)
		}),
	}
	err := healthCheckServer.ListenAndServe()
	if err != nil {
		log.Error("could not start health check server", "err", err)
	}
}