We can use a fixed value and close the channel if they don't cover their
amount; this wasn't really helping with anything other than setting a
floor for an expected feerate
Greatly simplify the changeset API. Instead of 'diff' we simply generate
the changes.
Also pulls up the 'next message' method, as at some point the
interactive tx protocol will be used for other things as well
(splices/closes etc)
Suggested-By: @rustyrussell
