Add state caching and invalidation to service type

This required a change in the event system to add an "activity" field.
This is meant to be generic in the case that there is more than one need
for it, but at the moment, allows a poke to tell that it is a poke in
response to an apply that just finished, instead of a regular poke or
backpoke in which all that matters is timestamp updates, because there
wasn't any actual work done (since that state was okay).

config.go

@@ -205,7 +205,7 @@ func UpdateGraphFromConfig(config *graphConfig, hostname string, g *Graph, etcdO
 	for _, v := range g.GetVertices() {
 		if !HasVertex(v, keep) {
 			// wait for exit before starting new graph!
-			v.Type.SendEvent(eventExit, true)
+			v.Type.SendEvent(eventExit, true, false)
 			g.DeleteVertex(v)
 		}
 	}