In this commit, we migrate all wire messages in the database from the
`legacy` to the `current` encoding.
This affects the way we write the `CommitDiff` and `LogUpdates` struct
to disk. We also need to migrate the network results bucket in the
switch as it includes a wire message without a length prefix.
We copy the new serialization togic to a new package 'current' and
export the methods we will need from the migration. This ensures that we
have isolation between the legacy and current serialization methods,
cleanly inidcating what type we are using during the migration.
The legacy encoding depends on the lnwire21 version of lnwire, so it will
let us change lnwire after the migration. To make sure it is separated
from the new encoding, we add it to a new package 'legacy'.
We also put common types in a new package 'common', which will house
types that won't change during the migration, and can be used by both
legacy and current serialization code.
