MEDIUM: watcher: fix race condition & plumbing stop for test

[amelhusic]

this fix is related to: https://github.com/haproxytech/haproxy-consul-connect/issues/10

[ShimmerGlass]

This will not stop the goroutines from running once the Watcher is stopped. One way to do that is to add a check in the infinite loops to stop them when the Watcher is stopped:

for {
  select {
    case <-w.shutdownCh:
      return
    default:
  }

  // work
}

Additionally, I'd prefer exposing a Stop() method rather than the chan directly.

[amelhusic]

I applied your recommendations.

[aiharos]

Please recheck and give a thumbs up for mergeing.