It used to always cycle all the peers when checking which one needed to
be notified, now it only does that when the target is multiple (i.e.
broadcast or exclusion).
Now uses two interfaces so it can be overridden in the future, and
core no longer depends on Node.
The interfaces are implements in scene/multiplayer.
Replaces root_node with root_path.
Remove all Node references from MultiplayerAPI.