lnwallet/chancloser: update co-op close state machine to be taproot aware

In this commit, we update the co-op close state machine to be proper
taproot aware. This involves setting the new nonce and partial
signatures fields in the shutdown and closing signed messages. The other
big change here is that we'll short circuit most of the co-op close
flow. Rather than attempt a "negotiation", we'll just have the responder
accept w/e the initiator sends, as they're the ones that pay the fees in
the end.

Author: Roasbeef