Remember, and attempt to set up later, `orphanRelations`.

These are relations where `relatedModel` can't be resolved when `setup()` runs.
In some cases, this could lead to relations never being initialized.
Optimization: bind directly `this.instance.on('destroy')`.

Well, more getting rid of a dumb construction... do this instead
of the weird workaround that was implemented; binding to
`Backbone.Relational.store.getCollection( this.instance )`,
then checking if `this.instance === model`.