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).

event.go

@@ -25,14 +25,15 @@ const (
 	eventStart
 	eventPause
 	eventPoke
-	eventChanged
+	eventBackPoke
 )
 
 type Event struct {
 	Name eventName
 	Resp chan bool // channel to send an ack response on, nil to skip
 	//Wg   *sync.WaitGroup // receiver barrier to Wait() for everyone else on
-	Msg string // some words for fun
+	Msg      string // some words for fun
+	Activity bool   // did something interesting happen?
 }
 
 // send a single acknowledgement on the channel if one was requested
@@ -47,3 +48,8 @@ func (event *Event) NACK() {
 		event.Resp <- false // send NACK
 	}
 }
+
+// get the activity value
+func (event *Event) GetActivity() bool {
+	return event.Activity
+}