Issue #68: Refactor DatabaseRowsetAbstract
Issue #68: Refactor DatabaseRowsetAbstract

- Do not extend from ObjectSet, compose it
- Implement missing interface methods
- Refactor implemetation of filter() method
- Change toArray() to return the current entity in the set

BREAKING! DatabaseRowsetAbstract::toArray() no longer flattens the whole entity collection it only returns a array representation of the current entity. To migrate your code, use iterator_to_array if you want to get a array representation of the all entities in the collection, or traverse through the collection.

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