R1:0446e636ea30

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 [[ http://www.phabricator.com/docs/phabricator/article/Performance_N+1_Query_Problem.html | 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…