R1:516aaad3410c

Use "pathIndex" in some owners package queries to improve query plans

Summary: Depends on D19184. Ref T11015. Now that we have a digest index column, we can improve some of the queries a bit.

Test Plan:
- Ran queries from revision pages before and after with and without EXPLAIN.
- Saw the same results with much better EXPLAIN plans.
- Fragment size is now fixed at 12 bytes per fragment, so we can shove more of them in a single query.

Maniphest Tasks: T11015

Differential Revision: https://secure.