Ensure chain notifier is started before accessed.

The use case comes from the RPC layer that is ready before the
chain notifier which is used in the sub server.

htlcswitch/mock.go

@@ -931,6 +931,10 @@ func (m *mockNotifier) Start() error {
 	return nil
 }
 
+func (m *mockNotifier) Started() bool {
+	return true
+}
+
 func (m *mockNotifier) Stop() error {
 	return nil
 }