feature: add a default feature bit for the spec's state machine

This commit adds a feature bit for the recently implemented state
machine as defined within the specification. With this commit, older
non-upgraded lnd nodes will fail a bit more gracefully when connecting
to updated nodes.

features.go

@@ -8,4 +8,6 @@ var globalFeatures = lnwire.NewFeatureVector([]lnwire.Feature{})
 
 // localFeatures is an feature vector which represent the features which
 // only affect the protocol between these two nodes.
-var localFeatures = lnwire.NewFeatureVector([]lnwire.Feature{})
+var localFeatures = lnwire.NewFeatureVector([]lnwire.Feature{
+	{"lcp-stop-and-wait", lnwire.RequiredFlag},
+})