`NodeTreeRef` is a thin wrapper on top of `bNodeTree`. By default it
should not hide anything from the underlying `bNodeTree` (before this
it was hiding muted links).
For convenience some "derived" data is cached on sockets. For example
all the connected sockets when reroutes and muted links are ignored.
A nice side benefit of this refactor is that `NodeTreeRef` requires
less memory than before.