In this commit, we add a new FeeRange TLV to the ClosingSigned message
that allows either side to attempt to cut the fee negotiation process
short by sending their min+max fee. In the ideal case, the funder sends
over their fee range, which is accepted by the responder. However, it's
possible for things to break down which results in either the close
attempting being abandoned, or they fall back to the legacy less defined
negotiation process.

In this commit, we begin to use the new fee rate information as a hint
to allow us to short circuit the co-op close negotiation process by
simply echo'ing back the initiator's fee rate if it's within our desired
range, or accepting the responder's fee rate for the initiator.

This is just an initial draft, as we still need to hone into exactly
what fee range we want to advertise as the initiator.