This adds the new range-based closing_signed negotiation specified
in lightning/bolts#847 as
well as cleans up the existing closing_signed negotiation to unify
the new codepaths and the old ones.

Note that because the new range-based closing_signed negotiation
allows the channel fundee to ultimately select the fee out of a
range specified by the funder, which we, of course, always select
the highest allowed amount from. Thus, we've added an extra round
of closing_signed in the common case as we will not simply accept
the first fee we see, always preferring to make the funder pay as
much as they're willing to.

Probably needs a few additional tests, but the existing shutdown coverage isn't bad either. May need to wait on resolution of a few issues in the spec PR.