Route Blinding

Route blinding allows a recipient to provide a blinded route to
potential payers. Each node_id in the route is tweaked, and dummy
hops may be included.

This is an alternative to rendezvous to preserve recipient anonymity.
It has a different set of trade-offs: onions are re-usable, but the
privacy guarantees are a bit weaker and require more work (e.g. when
handling payment fees and errors).