Make AbstractType.isByteOrderComparable abstract

I can't recall precisely what was agreed at the NGCC, but I'm reasonably sure we agreed to make this method abstract, put some javadoc explaining we may require fields to yield true in the near future, and potentially log a warning on startup if a user-defined type returns false.

This should make it into 3.0, IMO, so that we can look into migrating to byte-order comparable types in the post-3.0 world.