multi: add interface for subscribe client so it can be mocked

The current implementation of subscribe is difficult to mock because
the queue that we send updates on in unexported, so you cannot create
a subscribe.Client object and then add your own updates. While it is
possible to run a subscribe server in tests, subscribe servers will
shutdown before dispatching their udpates to all clients, which can be
flakey (and is difficult to workaround). In this commit, we add a
subscription interface so that these testing struggles can be addressed
with a mock.

subscribe/subscribe.go

@@ -14,9 +14,9 @@ var ErrServerShuttingDown = errors.New("subscription server shutting down")
 
 // Client is used to get notified about updates the caller has subscribed to,
 type Client struct {
-	// Cancel should be called in case the client no longer wants to
+	// cancel should be called in case the client no longer wants to
 	// subscribe for updates from the server.
-	Cancel func()
+	cancel func()
 
 	updates *queue.ConcurrentQueue
 	quit    chan struct{}
@@ -34,6 +34,12 @@ func (c *Client) Quit() <-chan struct{} {
 	return c.quit
 }
 
+// Cancel should be called in case the client no longer wants to
+// subscribe for updates from the server.
+func (c *Client) Cancel() {
+	c.cancel()
+}
+
 // Server is a struct that manages a set of subscriptions and their
 // corresponding clients. Any update will be delivered to all active clients.
 type Server struct {
@@ -118,7 +124,7 @@ func (s *Server) Subscribe() (*Client, error) {
 	client := &Client{
 		updates: queue.NewConcurrentQueue(20),
 		quit:    make(chan struct{}),
-		Cancel: func() {
+		cancel: func() {
 			select {
 			case s.clientUpdates <- &clientUpdate{
 				cancel:   true,