peer: conditionally create rbf coop close fsm based on feature bits

In this commit, we fully integrate the new RBF close state machine into
the peer.

For the restart case after shutdown, we can short circuit the existing
logic as the new FSM will handle retransmitting the shutdown message
itself, and doesn't need to delegate that duty to the link.

Unlike the existing state machine, we're able to restart the flow to
sign a coop close with a new higher fee rate. In this case, we can now
send multiple updates to the RPC caller, one for each newly singed coop
close transaction.

To implement the async flush case, we'll launch a new goroutine to wait
until the state machine reaches the `ChannelFlushing` state, then we'll
register the hook. We don't do this at start up, as otherwise the
channel may _already_ be flushed, triggering an invalid state
transition.

peer/daemon_adapters.go

@@ -23,8 +23,6 @@ type MessageSender interface {
 
 // flexMessageSender is a message sender-like interface that is aware of
 // sync/async semantics, and is bound to a single peer.
-//
-//nolint:unused
 type flexMessageSender interface {
 	// SendMessage sends a variadic number of high-priority messages to the
 	// remote peer. The first argument denotes if the method should block
@@ -36,16 +34,12 @@ type flexMessageSender interface {
 // peerMsgSender implements the MessageSender interface for a single peer.
 // It'll return an error if the target public isn't equal to public key of the
 // backing peer.
-//
-//nolint:unused
 type peerMsgSender struct {
 	sender  flexMessageSender
 	peerPub btcec.PublicKey
 }
 
 // newPeerMsgSender creates a new instance of a peerMsgSender.
-//
-//nolint:unused
 func newPeerMsgSender(peerPub btcec.PublicKey,
 	msgSender flexMessageSender) *peerMsgSender {
 
@@ -59,8 +53,6 @@ func newPeerMsgSender(peerPub btcec.PublicKey,
 //
 // TODO(roasbeef): current impl bound to single peer, need server pointer
 // otherwise?
-//
-//nolint:unused
 func (p *peerMsgSender) SendMessages(pub btcec.PublicKey,
 	msgs []lnwire.Message) error {