Composite generation should only pull fields from Mongo that are defined in specification

[rsinger]

When generating a Composite (Table row, View, Search document), it brings back the entire document in every query (join, etc.). While this likely isn't an issue on the Mongo end (maybe, though!), it's an inefficient use resources on the PHP side (memory, working with a much larger array than necessary, etc.).

If the Mongo query only returned the fields that are defined in the spec, this all becomes a lot more efficient.