resources: Remove SetWatching/IsWatching code from Watch

This removes some boilerplate from the Watch methods which can be baked
into the engine instead.

This code should be checked for races and locks to make sure we only
start resources when it makes sense to.

resources/pkg.go

@@ -109,11 +109,6 @@ func (obj *PkgRes) Validate() error {
 // TODO: https://github.com/hughsie/PackageKit/issues/109
 // TODO: https://github.com/hughsie/PackageKit/issues/110
 func (obj *PkgRes) Watch(processChan chan event.Event) error {
-	if obj.IsWatching() {
-		return nil
-	}
-	obj.SetWatching(true)
-	defer obj.SetWatching(false)
 	cuid := obj.Converger() // get the converger uid used to report status
 
 	bus := packagekit.NewBus()