discovery+peer: add logs to reveal shutdown flow

Also adds a `TODO` for checking the err chan.

peer/brontide.go

@@ -1401,6 +1401,8 @@ func newChanMsgStream(p *Brontide, cid lnwire.ChannelID) *msgStream {
 // channel announcements.
 func newDiscMsgStream(p *Brontide) *msgStream {
 	apply := func(msg lnwire.Message) {
+		// TODO(yy): `ProcessRemoteAnnouncement` returns an error chan
+		// and we need to process it.
 		p.cfg.AuthGossiper.ProcessRemoteAnnouncement(msg, p)
 	}