Rewrite query handling so that it also can be performed on fetched documents

A NoSQL query is currently converted into a WHERE-clause. The same functionality is used in the $match aggregation stage, which means that $match can only appear in the beginning of the aggregation pipeline.

The functionality should be rewritten, so that the matching can also be performed against documents fetched from the database. That way, the $match stage can appear anywhere in the aggregation pipeline. Regular finds would benefit as well, as it would allow some part of the matching to be performed using a WHERE-clause and some after the documents have been fetched from the database.