Storage format: Entity vs List<Eav>

[d-r-q]

Actual behavior: trx log is stored as List[Eav].

This solution has several disadvantages:

1. Redundant storage of Gids and attribute names
2. ~Unclear how to implement (nested) Map attributes~
3. Unclear how to support empty lists

As an alternative, trx may be stored as List[Entity]. Consider what is disadvantages of this solution and choose and implement one.

Probably it's worth to store trx as Map[EntityCollection,List[Entity]] (see #51).

[d-r-q]

Related to #8