lnwire: prep wire messages for TLV extensions

Messages:
- UpdateFulfillHTLC
- UpdateFee
- UpdateFailMalformedHTLC
- UpdateFailHTLC
- UpdateAddHTLC
- Shutdown
- RevokeAndAck
- ReplyShortChanIDsEnd
- ReplyChannelRange
- QueryShortChanIDs
- QueryChannelRange
- NodeAnnouncement
- Init
- GossipTimestampRange
- FundingSigned
- FundingLocked
- FundingCreated
- CommitSig
- ClosingSigned
- ChannelUpdate
- ChannelReestablish
- ChannelAnnouncement
- AnnounceSignatures

lnwire: update quickcheck tests, use constant for Error

multi: update unit tests to pass deep equal assertions with messages

In this commit, we update a series of unit tests in the code base to now
pass due to the new wire message encode/decode logic. In many instances,
we'll now manually set the extra bytes to an empty byte slice to avoid
comparisons that fail due to one message having an empty byte slice and
the other having a nil pointer.

lnwire/query_short_chan_ids.go

@@ -81,6 +81,11 @@ type QueryShortChanIDs struct {
 	// ShortChanIDs is a slice of decoded short channel ID's.
 	ShortChanIDs []ShortChannelID
 
+	// ExtraData is the set of data that was appended to this message to
+	// fill out the full maximum transport message size. These fields can
+	// be used to specify optional data such as custom TLV fields.
+	ExtraData ExtraOpaqueData
+
 	// noSort indicates whether or not to sort the short channel ids before
 	// writing them out.
 	//
@@ -114,8 +119,11 @@ func (q *QueryShortChanIDs) Decode(r io.Reader, pver uint32) error {
 	}
 
 	q.EncodingType, q.ShortChanIDs, err = decodeShortChanIDs(r)
+	if err != nil {
+		return err
+	}
 
-	return err
+	return q.ExtraData.Decode(r)
 }
 
 // decodeShortChanIDs decodes a set of short channel ID's that have been
@@ -292,7 +300,12 @@ func (q *QueryShortChanIDs) Encode(w io.Writer, pver uint32) error {
 
 	// Base on our encoding type, we'll write out the set of short channel
 	// ID's.
-	return encodeShortChanIDs(w, q.EncodingType, q.ShortChanIDs, q.noSort)
+	err = encodeShortChanIDs(w, q.EncodingType, q.ShortChanIDs, q.noSort)
+	if err != nil {
+		return err
+	}
+
+	return q.ExtraData.Encode(w)
 }
 
 // encodeShortChanIDs encodes the passed short channel ID's into the passed
@@ -425,5 +438,5 @@ func (q *QueryShortChanIDs) MsgType() MessageType {
 //
 // This is part of the lnwire.Message interface.
 func (q *QueryShortChanIDs) MaxPayloadLength(uint32) uint32 {
-	return MaxMessagePayload
+	return MaxMsgBody
 }