chainntnfs: acquire TxNotifier lock after closing quit chan

This prevents a deadlock while tearing down the TxNotifier if it's
currently blocked delivering a notification. By closing the quit chan
first, we ensure blocked sends/reads can exit and allow the TxNotifier
to proceed tearing down.

chainntnfs/txnotifier.go

@@ -1912,11 +1912,11 @@ func (n *TxNotifier) dispatchSpendReorg(ntfn *SpendNtfn) error {
 // closes the event channels of all registered notifications that have not been
 // dispatched yet.
 func (n *TxNotifier) TearDown() {
+	close(n.quit)
+
 	n.Lock()
 	defer n.Unlock()
 
-	close(n.quit)
-
 	for _, confSet := range n.confNotifications {
 		for _, ntfn := range confSet.ntfns {
 			close(ntfn.Event.Confirmed)