Issue #53: NullCheckFilter with an AND option

Introducing EvaluationMode parameter for NullCheckFilter with ANY_FIELD and ALL_FIELDS values. Query optimization is only available in the old ANY_FIELD mode. Optimization for ALL_FIELDS mode can be added later.

Closes #53.
I think we should re-phrase the shown names here. To:

 * When all fields are null, the record is considered NULL
 * When any field is null, the record is considered NULL