Many index implementations construct BSON objects to query indexes .

The process of creating a BSONObj query with a discriminator also encodes a 'l' or 'g' field
name, which is eventually encoded into KeyString. KeyString is capable of encoding this information without an 'l' or 'g', so we should transition the index implementations to convert directly to KeyString.

When SERVER-41720 is complete, it will be possible to make this transition for all storage engines except for ephermalForTest.