下列常量只在返回给客户端列式协议时使用.

边终点id或点id.

边终点type或点type.

the relative location of these three fields should be synced with DirectionType.

<int64, int32>.

<string, string>.

These three types are not in the data type protocol with server side, start from 1024 should be enough for future new server types.

ValueType 用于到客户端的列式协议中，客户端识别反序列化成Property还是具体的Int32、Double等类型.

TODO(huyingqian): support Label.