lnwire: Add upfront shutdown messages and feature bit

This commit adds the feature bit and additional fields
required in `open_channel` and `accept_channel` wire
messages for `option_upfront_shutdown_script`.

lnwire/lnwire.go

@@ -816,8 +816,8 @@ func ReadElement(r io.Reader, element interface{}) error {
 		}
 		length := binary.BigEndian.Uint16(addrLen[:])
 
-		var addrBytes [34]byte
-		if length > 34 {
+		var addrBytes [deliveryAddressMaxSize]byte
+		if length > deliveryAddressMaxSize {
 			return fmt.Errorf("Cannot read %d bytes into addrBytes", length)
 		}
 		if _, err = io.ReadFull(r, addrBytes[:length]); err != nil {