Currently the underlying array backing the hop's TLVRecords is modified
when combining custom records with the primitive forwarding info. This
commit uses a fresh slice to prevent modifications from mutating the
hop itself.
In this commit, we extend the Hop struct to carry an arbitrary set of
TLV values, and add a new field that allows us to distinguish between
the modern and legacy TLV payload.
We add a new `PackPayload` method that will be used to encode the
combined required routing TLV fields along any set of TLV fields that
were specified as part of path finding.
Finally, the `ToSphinxPath` has been extended to be able to recognize if
a hop needs the modern, or legacy payload.
In this commit, we update the process that we use to generate a sphinx
packet to send our onion routed HTLC. Due to recent changes in the
`sphinx` package we use, we now need to use a new PaymentPath struct. As
a result, it no longer makes sense to split up the nodes in a route and
their per hop paylods as they're now in the same struct. All tests have
been updated accordingly.
