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/interface.go

@@ -0,0 +1,17 @@
+package subscribe
+
+// Subscription is an interface implemented by subscriptions to a server
+// providing updates.
+type Subscription interface {
+	// Updates returns a read-only channel where the updates the client has
+	// subscribed to will be delivered.
+	Updates() <-chan interface{}
+
+	// Quit is a channel that will be closed in case the server decides to
+	// no longer deliver updates to this client.
+	Quit() <-chan struct{}
+
+	// Cancel should be called in case the client no longer wants to
+	// subscribe for updates from the server.
+	Cancel()
+}