code: public static Message buildCommit(Zxid zxid) {
      ZabMessage.Zxid cZxid = toProtoZxid(zxid);
  
      Commit commit = Commit.newBuilder().setZxid(cZxid).build();
  
      return Message.newBuilder().setType(MessageType.COMMIT)
                                 .setCommit(commit)
                                 .build();
    }
docs: Creates a COMMIT message.
  
  @param zxid the id of the committed transaction.
  @return a protobuf message.