FTS adds all documents it receives from KV to the _id field

FTS adds all document keys to the internal `_id` field of the index. This is something the user cannot control.

As documented in CBSE-10836, this may not be the best design in some cases where the user wants to run negate queries - during which the match_all query is conjuncted with the must_not criteria to return only documents that DO NOT qualify the must_not criteria.

The match_all query runs over the `_id` field - which means if the user had defined an index only over a certain type, documents of all other types become eligible for the query as well - which may not be desirable in a few situations.

The request here is to improve this area to store the "_id" of only those documents that are eligible for the type mapping(s).