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).

service.go

@@ -82,9 +82,10 @@ func (obj *ServiceType) Watch() {
 
 	var service = fmt.Sprintf("%v.service", obj.Name) // systemd name
 	var send = false                                  // send event?
-	var invalid = false                               // does the service exist or not?
-	var previous bool                                 // previous invalid value
-	set := conn.NewSubscriptionSet()                  // no error should be returned
+	var dirty = false
+	var invalid = false              // does the service exist or not?
+	var previous bool                // previous invalid value
+	set := conn.NewSubscriptionSet() // no error should be returned
 	subChannel, subErrors := set.Subscribe()
 	var activeSet = false
 
@@ -112,6 +113,7 @@ func (obj *ServiceType) Watch() {
 
 		if previous != invalid { // if invalid changed, send signal
 			send = true
+			dirty = true
 		}
 
 		if invalid {
@@ -133,6 +135,9 @@ func (obj *ServiceType) Watch() {
 				if ok := obj.ReadEvent(&event); !ok {
 					return // exit
 				}
+				if event.GetActivity() {
+					dirty = true
+				}
 				send = true
 			case _ = <-TimeAfterOrBlock(obj.ctimeout):
 				obj.SetConvergedState(typeConvergedTimeout)
@@ -166,6 +171,7 @@ func (obj *ServiceType) Watch() {
 					log.Printf("Service[%v]->Stopped", service)
 				}
 				send = true
+				dirty = true
 
 			case err := <-subErrors:
 				obj.SetConvergedState(typeConvergedNil) // XXX ?
@@ -178,12 +184,19 @@ func (obj *ServiceType) Watch() {
 				if ok := obj.ReadEvent(&event); !ok {
 					return // exit
 				}
+				if event.GetActivity() {
+					dirty = true
+				}
 				send = true
 			}
 		}
 
 		if send {
 			send = false
+			if dirty {
+				dirty = false
+				obj.isStateOK = false // something made state dirty
+			}
 			Process(obj) // XXX: rename this function
 		}
 
@@ -191,6 +204,9 @@ func (obj *ServiceType) Watch() {
 }
 
 func (obj *ServiceType) StateOK() bool {
+	if obj.isStateOK { // cache the state
+		return true
+	}
 
 	if !util.IsRunningSystemd() {
 		log.Fatal("Systemd is not running.")