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.

msgmux/msg_router.go

@@ -1,6 +1,7 @@
 package msgmux
 
 import (
+	"context"
 	"fmt"
 	"maps"
 	"sync"
@@ -46,7 +47,7 @@ type Endpoint interface {
 
 	// SendMessage handles the target message, and returns true if the
 	// message was able being processed.
-	SendMessage(msg PeerMsg) bool
+	SendMessage(ctx context.Context, msg PeerMsg) bool
 }
 
 // MsgRouter is an interface that represents a message router, which is generic
@@ -66,7 +67,7 @@ type Router interface {
 	RouteMsg(PeerMsg) error
 
 	// Start starts the peer message router.
-	Start()
+	Start(ctx context.Context)
 
 	// Stop stops the peer message router.
 	Stop()
@@ -137,12 +138,12 @@ func NewMultiMsgRouter() *MultiMsgRouter {
 }
 
 // Start starts the peer message router.
-func (p *MultiMsgRouter) Start() {
+func (p *MultiMsgRouter) Start(ctx context.Context) {
 	log.Infof("Starting Router")
 
 	p.startOnce.Do(func() {
 		p.wg.Add(1)
-		go p.msgRouter()
+		go p.msgRouter(ctx)
 	})
 }
 
@@ -179,7 +180,7 @@ func (p *MultiMsgRouter) endpoints() fn.Result[EndpointsMap] {
 }
 
 // msgRouter is the main goroutine that handles all incoming messages.
-func (p *MultiMsgRouter) msgRouter() {
+func (p *MultiMsgRouter) msgRouter(ctx context.Context) {
 	defer p.wg.Done()
 
 	// endpoints is a map of all registered endpoints.
@@ -235,7 +236,7 @@ func (p *MultiMsgRouter) msgRouter() {
 						"msg %T to endpoint %s", msg,
 						endpoint.Name())
 
-					sent := endpoint.SendMessage(msg)
+					sent := endpoint.SendMessage(ctx, msg)
 					couldSend = couldSend || sent
 				}
 			}