Create an event bus

[dkropachev]

To implement #616, we need an event bus in the driver that lets us listen to events originating either from the driver itself or from the cluster

API

Subscribe/Unsubscribe:

	// Subscribe
	sub := sess.SubscribeToEvents("my-subs", maxQueueSize, nil)
	// Unsubscribe
	defer sub.Stop()

Filtering:

	sub := sess.SubscribeToEvents("schema-event", maxQueueSize, func(ev events.Event) bool {
		return ev.Type() == events.ClusterEventTypeSchemaChangeKeyspace
	})

[mykaul]

I'd be happy to see the design for this.

[dkropachev]

I'd be happy to see the design for this.

Updated issue description