Unify blinding nomenclature to call them "paths" not "routes".

Unify blinding nomenclature to call them "paths" not "routes".

Currently the `onion_message` module exposes the blinded route
object as *both* `BlindedRoute` and `BlindedPath`. This is somewhat
confusing, and given they are really paths, not routes (at least in
the sense that a route could be multi-path, though for OMs they are
not), here we unify to only call them paths.