resources: Update state checks

The mgmt graph depends on state tracking to eliminate redundant pokes.
With the Watch loop now able to produce events quickly, it should no
longer play a part in determining the vertex state. This simplifies the
resource API as well!

docs/resource-guide.md

@@ -333,7 +333,6 @@ func (obj *FooRes) Watch(processChan chan *event.Event) error {
 	var send = false // send event?
 	var exit *error
 	for {
-		obj.SetState(ResStateWatching) // reset
 		select {
 		case event := <-obj.Events():
 			cuid.SetConverged(false)