checkpoint on persistent queue update

lib/chat/cubits/single_contact_messages_cubit.dart

@@ -107,6 +107,7 @@ class SingleContactMessagesCubit extends Cubit<SingleContactMessagesState> {
         table: 'SingleContactUnsentMessages',
         key: _remoteConversationRecordKey.toString(),
         fromBuffer: proto.Message.fromBuffer,
+        toBuffer: (x) => x.writeToBuffer(),
         closure: _processUnsentMessages,
         onError: (e, st) {
           log.error('Exception while processing unsent messages: $e\n$st\n');