routing: close exit chan before ntfnChan to cancel topology clients

This commit fixes a panic due to a send on a closed channel that could
possibly occur depending on the order of channel closes when a client
goes to cancel a topology notification client.

Previously we closed the ntfnChan first, this would possible result in
a panic as the goroutine may have succeeded on a send at the same time
the channel was closed. Instead, we now close the `exit` channel first
which is meant to be a signal to the goroutine that the client has been
canceled.

routing/router.go

@@ -489,8 +489,8 @@ func (r *ChannelRouter) networkHandler() {
 			if ntfnUpdate.cancel {
 				if client, ok := r.topologyClients[ntfnUpdate.clientID]; ok {
 					delete(r.topologyClients, clientID)
-					close(client.ntfnChan)
 					close(client.exit)
+					close(client.ntfnChan)
 				}
 
 				continue