resources: Improve notification system and notify refreshes

Resources can send "refresh" notifications along edges. These messages
are sent whenever the upstream (initiating vertex) changes state. When
the changed state propagates downstream, it will be paired with a
refresh flag which can be queried in the CheckApply method of that
resource.

Future work will include a stateful refresh tracking mechanism so that
if a refresh event is generated and not consumed, it will be saved
across an interrupt (shutdown) or a crash so that it can be re-applied
on the subsequent run. This is important because the unapplied refresh
is a form of hysteresis which needs to be tracked and remembered or we
won't be able to determine that the state is wrong!

Still to do:
* Update the autogrouping code to handle the edge notify properties!
* Actually finish the stateful bool code

examples/lib/libmgmt3.go

@@ -83,7 +83,20 @@ func (obj *MyGAPI) Graph() (*pgraph.Graph, error) {
 	v2 := pgraph.NewVertex(f1)
 	g.AddVertex(v2)
 
+	s1 := &resources.SvcRes{
+		BaseRes: resources.BaseRes{
+			Name: "purpleidea",
+		},
+		State: "stopped",
+	}
+
+	v3 := pgraph.NewVertex(s1)
+	g.AddVertex(v3)
+
 	g.AddEdge(v1, v2, pgraph.NewEdge("e1"))
+	e2 := pgraph.NewEdge("e2")
+	e2.Notify = true // send a notification from v2 to v3
+	g.AddEdge(v2, v3, e2)
 
 	//g, err := config.NewGraphFromConfig(obj.data.Hostname, obj.data.EmbdEtcd, obj.data.Noop)
 	return g, nil