lnpeer: extend Peer interface with new QuitSignal method

In this commit, we extend the Peer interface with a new QuitSignal
method. This method is meant to expose a read-only quit channel which
will allow callers to cancel any actions based on the lifetime of the
underlying peer.

lnpeer/peer.go

@@ -33,4 +33,10 @@ type Peer interface {
 
 	// Address returns the network address of the remote peer.
 	Address() net.Addr
+
+	// QuitSignal is a method that should return a channel which will be
+	// sent upon or closed once the backing peer exits. This allows callers
+	// using the interface to cancel any processing in the event the backing
+	// implementation exits.
+	QuitSignal() <-chan struct{}
 }