Use go-events package

TBD: Queue not converted yet

Signed-off-by: Elliot Pahl <elliot.pahl@gmail.com>

vendor/github.com/docker/go-events/README.md

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+# Docker Events Package
+
+[![GoDoc](https://godoc.org/github.com/docker/go-events?status.svg)](https://godoc.org/github.com/docker/go-events)
+[![Circle CI](https://circleci.com/gh/docker/go-events.svg?style=shield)](https://circleci.com/gh/docker/go-events)
+
+The Docker `events` package implements a composable event distribution package
+for Go.
+
+Originally created to implement the [notifications in Docker Registry
+2](https://github.com/docker/distribution/blob/master/docs/notifications.md),
+we've found the pattern to be useful in other applications. This package is
+most of the same code with slightly updated interfaces. Much of the internals
+have been made available.
+
+## Usage
+
+The `events` package centers around a `Sink` type.  Events are written with
+calls to `Sink.Write(event Event)`. Sinks can be wired up in various
+configurations to achieve interesting behavior.
+
+The canonical example is that employed by the
+[docker/distribution/notifications](https://godoc.org/github.com/docker/distribution/notifications)
+package. Let's say we have a type `httpSink` where we'd like to queue
+notifications. As a rule, it should send a single http request and return an
+error if it fails:
+
+```go
+func (h *httpSink) Write(event Event) error {
+	p, err := json.Marshal(event)
+	if err != nil {
+		return err
+	}
+	body := bytes.NewReader(p)
+	resp, err := h.client.Post(h.url, "application/json", body)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+	
+	if resp.Status != 200 {
+		return errors.New("unexpected status")
+	}
+
+	return nil
+}
+
+// implement (*httpSink).Close()
+```
+
+With just that, we can start using components from this package. One can call
+`(*httpSink).Write` to send events as the body of a post request to a
+configured URL.
+
+### Retries
+
+HTTP can be unreliable. The first feature we'd like is to have some retry:
+
+```go
+hs := newHTTPSink(/*...*/)
+retry := NewRetryingSink(hs, NewBreaker(5, time.Second))
+```
+
+We now have a sink that will retry events against the `httpSink` until they
+succeed. The retry will backoff for one second after 5 consecutive failures
+using the breaker strategy.
+
+### Queues
+
+This isn't quite enough. We we want a sink that doesn't block while we are
+waiting for events to be sent. Let's add a `Queue`:
+
+```go
+queue := NewQueue(retry)
+```
+
+Now, we have an unbounded queue that will work through all events sent with
+`(*Queue).Write`. Events can be added asynchronously to the queue without
+blocking the current execution path. This is ideal for use in an http request.
+
+### Broadcast
+
+It usually turns out that you want to send to more than one listener. We can
+use `Broadcaster` to support this:
+
+```go
+var broadcast = NewBroadcaster() // make it available somewhere in your application.
+broadcast.Add(queue) // add your queue!
+broadcast.Add(queue2) // and another!
+```
+
+With the above, we can now call `broadcast.Write` in our http handlers and have
+all the events distributed to each queue. Because the events are queued, not
+listener blocks another.
+
+### Extending
+
+For the most part, the above is sufficient for a lot of applications. However,
+extending the above functionality can be done implementing your own `Sink`. The
+behavior and semantics of the sink can be completely dependent on the
+application requirements. The interface is provided below for reference:
+
+```go
+type Sink {
+	Write(Event) error
+	Close() error
+}
+```
+
+Application behavior can be controlled by how `Write` behaves. The examples
+above are designed to queue the message and return as quickly as possible.
+Other implementations may block until the event is committed to durable
+storage.
+
+## Copyright and license
+
+Copyright © 2016 Docker, Inc. go-events is licensed under the Apache License,
+Version 2.0. See [LICENSE](LICENSE) for the full license text.