resources: Simplify resource Converger and Startup code

This takes the Converged initialization and Startup patterns that are
common in all resources, and bakes it into the core engine. This way
resource writing is much more concise and there is less boilerplate!

pgraph/actions.go

@@ -415,7 +415,9 @@ func (g *Graph) Worker(v *Vertex) error {
 		}
 
 		// TODO: reset the watch retry count after some amount of success
+		v.Res.RegisterConverger()
 		e := v.Res.Watch(processChan)
+		v.Res.UnregisterConverger()
 		if e == nil { // exit signal
 			err = nil // clean exit
 			break