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/update_fail_htlc.go

@@ -26,6 +26,11 @@ type UpdateFailHTLC struct {
 	// failed. This blob is only fully decryptable by the initiator of the
 	// HTLC message.
 	Reason OpaqueReason
+
+	// 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
 }
 
 // A compile time check to ensure UpdateFailHTLC implements the lnwire.Message
@@ -41,6 +46,7 @@ func (c *UpdateFailHTLC) Decode(r io.Reader, pver uint32) error {
 		&c.ChanID,
 		&c.ID,
 		&c.Reason,
+		&c.ExtraData,
 	)
 }
 
@@ -53,6 +59,7 @@ func (c *UpdateFailHTLC) Encode(w io.Writer, pver uint32) error {
 		c.ChanID,
 		c.ID,
 		c.Reason,
+		c.ExtraData,
 	)
 }
 
@@ -69,21 +76,7 @@ func (c *UpdateFailHTLC) MsgType() MessageType {
 //
 // This is part of the lnwire.Message interface.
 func (c *UpdateFailHTLC) MaxPayloadLength(uint32) uint32 {
-	var length uint32
-
-	// Length of the ChanID
-	length += 32
-
-	// Length of the ID
-	length += 8
-
-	// Length of the length opaque reason
-	length += 2
-
-	// Length of the Reason
-	length += 292
-
-	return length
+	return MaxMsgBody
 }
 
 // TargetChanID returns the channel id of the link for which this message is