Remove blank node expansion features

[kmcginnes]

The current blank node implementation is slowing down and delaying progress on core RDF features.

We should limit blank node support as much as needed to enable forward progress on core RDF functionality.

Options

There are a couple of paths forward to represent blank nodes. A path will be chosen based on which one clears the path for core RDF functionality progress.

1. Represent blank nodes as a node with no neighbors
2. Represent blank nodes as a property in the node details