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.

htlcswitch/payment_result_test.go

@@ -39,18 +39,21 @@ func TestNetworkResultSerialization(t *testing.T) {
 		ChanID:          chanID,
 		ID:              2,
 		PaymentPreimage: preimage,
+		ExtraData:       make([]byte, 0),
 	}
 
 	fail := &lnwire.UpdateFailHTLC{
-		ChanID: chanID,
-		ID:     1,
-		Reason: []byte{},
+		ChanID:    chanID,
+		ID:        1,
+		Reason:    []byte{},
+		ExtraData: make([]byte, 0),
 	}
 
 	fail2 := &lnwire.UpdateFailHTLC{
-		ChanID: chanID,
-		ID:     1,
-		Reason: reason[:],
+		ChanID:    chanID,
+		ID:        1,
+		Reason:    reason[:],
+		ExtraData: make([]byte, 0),
 	}
 
 	testCases := []*networkResult{