store.RemoveNode should accept an IRI instead of opaque node

[oren]

store.RemoveNode(node) removes all quads which have the given node as subject, predicate, object, or label. Currently it accepts an opaque node.

For example:

idIRI := quad.IRI(id)
node := store.ValueOf(id)
err := store.RemoveNode(node)

(store.ValueOf returns an opaque node)

We had a discussion about it on discourse and agreed that there is no need to expose the opaque node to the user and store.RemoveNode can accept IRI.
The new API will look like this:

idIRI := quad.IRI(id)
err := store.RemoveNode(idIRI)

This will help new users since they don't need to be aware of the lower level concept of opaque node.