We rename `ChanUpdateOptionMaxHtlc` to `ChanUpdateRequiredMaxHtlc`
as with the latest changes it is now required.
Similarly, rename `validateOptionalFields` to
`ValidateChannelUpdateFields`, export it to use it in a later commit.
To avoid code changing underneath the static migrations, we copy the
lnwire dependency in its current form into the migration directory.
Ideally the migrations doesn't depend on any code that might change,
this takes us a step closer.
