In this commit, we add a new state machine to the project in order to
simplify and encapsulate the logic of negotiating the fees of the
closing transaction during the cooperative channel closure procedure.
This new implementation fixes several defects in the prior version of
the fee negotiation protocol as previously implemented within the peer.
Namely, we’ll now ensure that we converge to an acceptable fee
eventually, and clamp down the fee in the case that the starting fee is
greater than the fee of the commitment transaction.
The new fee negotiation also is very simple and should serve as a good
base for future improvements. After proposing our initial fee, if the
fee proposed by the remote party isn’t within 30% of our last fee, then
we increase/decrease our fee by 10%. This process continues until
either they send the same fee as we sent in a prior round, or they send
the identical fee that we sent in the last round.
