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`.
So that we can use it in TLV encoding.
Also add this to the codec for channeldb migration 32 since we will be
using it there in an upcoming adjustment commit.
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.
In preparation for the commit which will add the main logic for
migration 32 (which will migrate the MC store to use a more minimal
encoding), this commit just adds some of the code that the migration
will need to the package.
