In this commit, we add a new `TestMessage` interface for use in property
tests. With this, we'll be able to generate a random instance of a given
message, using the rapid byte stream. This can also eventually be useful
for fuzzing.
Add the ability to specify messages < CustomRangeStart that will still
be treated like custom messages by lnd (rather than unknown ones). This
allows code external to lnd to handle protocol messages that are *not
yet known to lnd*.
