In this commit, we enable a custom payer for the rbf coop close. This
allows us to ensure that the party that started one side of the close
flow pays the fees.
In this commit, we add the state transitions for the new protofsm based
RBF chan closer. The underlying protocol is a new asymmetric co-op close
process, wherein either side can initiate a chan closer, and use their
settled funds to pay for fees within the channel.
