routing: wait for topology clients to fully exit before closing ntfn chan

This commit fixes a send on closed channel panic by adding additional
synchronization when cancelling the notifications for a particular
topology client. We now ensure that all goroutines belonging to a
particular topology client exit fully before we close the notification
channel in order to avoid a panic.

routing/router.go

@@ -489,7 +489,10 @@ func (r *ChannelRouter) networkHandler() {
 			if ntfnUpdate.cancel {
 				if client, ok := r.topologyClients[ntfnUpdate.clientID]; ok {
 					delete(r.topologyClients, clientID)
+
 					close(client.exit)
+					client.wg.Wait()
+
 					close(client.ntfnChan)
 				}