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).

pgraph.go

@@ -556,7 +556,7 @@ func (g *Graph) Start(wg *sync.WaitGroup) { // start or continue
 		}
 
 		// ensure state is started before continuing on to next vertex
-		v.Type.SendEvent(eventStart, true)
+		v.Type.SendEvent(eventStart, true, false)
 
 	}
 }
@@ -564,7 +564,7 @@ func (g *Graph) Start(wg *sync.WaitGroup) { // start or continue
 func (g *Graph) Pause() {
 	t, _ := g.TopologicalSort()
 	for _, v := range t { // squeeze out the events...
-		v.Type.SendEvent(eventPause, true)
+		v.Type.SendEvent(eventPause, true, false)
 	}
 }
 
@@ -576,7 +576,7 @@ func (g *Graph) Exit() {
 		// when we hit the 'default' in the select statement!
 		// XXX: we can do this to quiesce, but it's not necessary now
 
-		v.Type.SendEvent(eventExit, true)
+		v.Type.SendEvent(eventExit, true, false)
 	}
 }