In this commit, we update an existing migration which at the time of
writing has not been included in a release. We update it so that it
converts the format used for MissionControl result encoding to use pure
TLV instead. The 3 structs that have been updated are: `mcHop`,
`mcRoute` and `paymentResult`.
Add a new mcRoute type that houses the data about a route that MC
actually uses. Then add a migration (channeldb/migration32) that
migrates the existing store from its current serialisation to the new,
more minimal serialisation.
