Race in initial setting of backends

[KnicKnic]

Possible race condition in creating a server & updating backends.

Timeline

1. Create a server&start it
2. This eventually calls start in discovery.go
   1. In discovery.go, if we are using a backend with 0 timeout such as static files we set create a go routine that will set the initial backend list and exit.
   2. However since this is in a go routine, the initial write could be delayed till after someone programatically called UpdateBackends. The updates would race with the last writer winning.
      1. This is a bug if the UpdateBackends wins, and the initial value succeeds. This race does not exist after initial set, due to all UpdateBackends pushing into the channel synchronously.

gobetween/src/discovery/discovery.go

Lines 94 to 144 in 7d8a736

	func (this *Discovery) Start() {
	log := logging.For("discovery")
	this.out = make(chan []core.Backend)
	this.stop = make(chan bool)
	// Prepare interval
	interval, err := time.ParseDuration(this.cfg.Interval)
	if err != nil {
	log.Fatal(err)
	}
	// TODO: rewrite with channels for stop
	go func() {
	for {
	backends, err := this.fetch(this.cfg)
	select {
	case <-this.stop:
	log.Info("Stopping discovery ", this.cfg)
	return
	default:
	}
	if err != nil {
	log.Error(this.cfg.Kind, " error ", err, " retrying in ", this.opts.RetryWaitDuration.String())
	log.Info("Applying failpolicy ", this.cfg.Failpolicy)
	if this.cfg.Failpolicy == "setempty" {
	this.backends = &[]core.Backend{}
	if !this.send() {
	log.Info("Stopping discovery ", this.cfg)
	return
	}
	}
	if !this.wait(this.opts.RetryWaitDuration) {
	log.Info("Stopping discovery ", this.cfg)
	return
	}
	continue
	}
	// cache
	this.backends = backends
	if !this.send() {
	log.Info("Stopping discovery ", this.cfg)
	return
	}

[KnicKnic]

@yyyar I accidentally referenced this issue in pr #261 . However it was related to a different issue with the word race in it. This one should stay active till atleast #258 is merged (if not longer). I do not like that 258 still requires non obvious cooperations from the calling client.