feature: advertise required feature bit for TLV onions

In this commit, we start to advertise the required feature bit for the
new modern TLV onion format. In a future change, we'll remove the logic
for being able to encode+encode the payload. For now, we still have
tests that exercise being able to use the new and old formats (as well
as both of them in a single route), so we'll continue to retain that
behavior for now.

Implements lightning/bolts#962.

docs/release-notes/release-notes-0.15.0.md

@@ -1,5 +1,9 @@
 # Release Notes
 
+## P2P Protocol
+
+[lnd now advertises the TLV onion feature bit as required(https://github.com/lightningnetwork/lnd/pull/6336).
+
 ## Security
 
 * [Misconfigured ZMQ
@@ -179,4 +183,4 @@ gRPC performance metrics (latency to process `GetInfo`, etc)](https://github.com
 * Thebora Kompanioni
 * Torkel Rogstad
 * Vsevolod Kaganovych
-* Yong Yu
+* Yong Yu

feature/default_sets.go

@@ -18,7 +18,7 @@ var defaultSetDesc = setDesc{
 		SetInit:    {}, // I
 		SetNodeAnn: {}, // N
 	},
-	lnwire.TLVOnionPayloadOptional: {
+	lnwire.TLVOnionPayloadRequired: {
 		SetInit:         {}, // I
 		SetNodeAnn:      {}, // N
 		SetInvoice:      {}, // 9