This commit prepares for more manipulation of custom records. A list of
tlv.Record types is more difficult to use than the more basic
map[uint64][]byte.
Furthermore fields and variables are renamed to make them more
consistent.
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.
