Revisit Proposal trait for DA, VID

[ggutoski]

          It probably should, but also we need to revisit those traits - we are outgrowing them.  I ran into a similar issue with the timeout certificate logic where the exchange trait is too restrictive.  And the ViewSync logic works with those traits, but is a bit of a square peg in a round hole.

Originally posted by @elliedavidson in #1684 (comment)