[#1763] morph/event: Write log message about new notification event

Sometimes it is useful to track sidechain events processed by the node.
For example, we need some easy way to look up for log messages about new
epoch notifications.

Make `subscriber.Subscriber` to log names of all events received from
the sidechain notification channel.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/morph/subscriber/subscriber.go

@@ -133,6 +133,10 @@ func (s *subscriber) routeNotifications(ctx context.Context) {
 					continue
 				}
 
+				s.log.Debug("new notification event from sidechain",
+					zap.String("name", notifyEvent.Name),
+				)
+
 				s.notifyChan <- notifyEvent
 			case neorpc.BlockEventID:
 				b, ok := notification.Value.(*block.Block)