lnwire: Add BOLT#1 'init' message

'init' message is the first message reveals the features supported or required
by this node. Nodes wait for receipt of the other's features to simplify error
diagnosis where features are incompatible. This message will help negotioate the
the features which are supported by both sides.

lnwire/lnwire.go

@@ -282,6 +282,10 @@ func writeElement(w io.Writer, element interface{}) error {
 		if _, err := w.Write(idx[:]); err != nil {
 			return err
 		}
+	case *FeatureVector:
+		if err := e.Encode(w); err != nil {
+			return err
+		}
 	case *wire.OutPoint:
 		// TODO(roasbeef): consolidate with above
 		// First write out the previous txid.
@@ -455,6 +459,14 @@ func readElement(r io.Reader, element interface{}) error {
 			return err
 		}
 		*e = pubKey
+	case **FeatureVector:
+		f, err := NewFeatureVectorFromReader(r)
+		if err != nil {
+			return err
+		}
+
+		*e = f
+
 	case *[]uint64:
 		var numItems uint16
 		if err := readElement(r, &numItems); err != nil {