Fetching results moved from datastore into cursor.

Just refactoring, no new functionality.
Fixed bug in filtering by repeated property which is projected.

Warning: filtering values of projected repeated property is made on application layer,
because splitting the entities while handling projection on repeated prop is also made
in the stub. There's probably no way how to do it on the side of Mongo.
Added tests for deleting property value. Passed.
Added tests for queries filtering by equality to None.