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/reply_short_chan_ids_end.go

@@ -22,6 +22,11 @@ type ReplyShortChanIDsEnd struct {
 	// set of short chan ID's in the corresponding QueryShortChanIDs
 	// message.
 	Complete uint8
+
+	// 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
 }
 
 // NewReplyShortChanIDsEnd creates a new empty ReplyShortChanIDsEnd message.
@@ -41,6 +46,7 @@ func (c *ReplyShortChanIDsEnd) Decode(r io.Reader, pver uint32) error {
 	return ReadElements(r,
 		c.ChainHash[:],
 		&c.Complete,
+		&c.ExtraData,
 	)
 }
 
@@ -52,6 +58,7 @@ func (c *ReplyShortChanIDsEnd) Encode(w io.Writer, pver uint32) error {
 	return WriteElements(w,
 		c.ChainHash[:],
 		c.Complete,
+		c.ExtraData,
 	)
 }
 
@@ -69,6 +76,5 @@ func (c *ReplyShortChanIDsEnd) MsgType() MessageType {
 //
 // This is part of the lnwire.Message interface.
 func (c *ReplyShortChanIDsEnd) MaxPayloadLength(uint32) uint32 {
-	// 32 (chain hash) + 1 (complete)
-	return 33
+	return MaxMsgBody
 }