Add an abstraction for preventing the 1+N queries problem

Summary:
Our code is quite complex in areas where we prevents the 1+N queries problem explained in a performance chapter.

This diff adds an abstraction for preventing this code.

Test Plan:
Run all examples mentioned in the doc-comments with logging the queries.
Generate and read docs.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2557