Make `DataLossProtect` fields required and remove wrappers

Make `DataLossProtect` fields required and remove wrappers

The fields provided by `DataLossProtect` have been mandatory since
https://github.com/lightning/bolts/pull/754/commits/6656b70, regardless
of whether `option_dataloss_protect` or `option_remote_key` feature bits
are set.

We move the fields out of `DataLossProtect` to make encoding definitions
more succinct with `impl_writeable_msg!` and to reduce boilerplate.

This paves the way for completely removing `OptionalField` in subsequent
commits.