Consider forbidding user to specify Gids

[d-r-q]

Actual behavior: user can specify gid while creating DetachedEntity or typed object

This approach is error prone, since user break gids generation logic.

Xodus doesn't allow users to manually create entity objects, users should ask Db to create new entity. This approach solves problem of gids generation, but currently it's unclear how to implement such use case with this approach:

1. User has some kind of web service
2. From client users is receiving updates for entities in JSON format.
3. How then user can update entity with this data? Is he should to make additional request to fetch entity, update this entity with incoming data and persist it?

Related: #27 #61