discovery: select on peer's QuitSignal to allow caller to unblock if disconnecting

In this commit, we select on the peer's QuitSignal to allow the caller
to unblock if the peer itself is disconnecting. With this change, we now
ensure that it isn't possible for a peer to block on this method and
prevent a graceful exit.

Author: Roasbeef

discovery/gossiper.go

@@ -481,6 +481,11 @@ func (d *AuthenticatedGossiper) ProcessRemoteAnnouncement(msg lnwire.Message,
 
 	select {
 	case d.networkMsgs <- nMsg:
+
+	// If the peer that sent us this error is quitting, then we don't need
+	// to send back an error and can return immediately.
+	case <-peer.QuitSignal():
+		return nil
 	case <-d.quit:
 		nMsg.err <- ErrGossiperShuttingDown
 	}