In this commit, we extend the `BuildRoute` method and RPC on the router
sub-server to accept a raw payment address which will be included as
part of an MPP payload for the finla hop. This change actually also
allows users to craft their own MPP paths using BuildRoute+SendToRoute.
Our primary goal however, was to fix some broken itests since we now
require the payAddr to be present for ALL payments other than key send
payments.
In this commit we add the ability to intercept forwarded htlc packets
straight from the RPC layer. The RPC layer handles a bidrectional stream
that comminucates to the client the intercepted packets and handles its
response by coordinating with the interceptable switch.
