Add a missing table key to improve performance of "Recently Completed Tasks" query

Summary:
Fixes T11490. Currently, this query can not use a key and the table size may be quite large.

Adjust the query so it can use a key for both selection and ordering, and add that key.

Test Plan: Ran EXPLAIN on the old query in production, then added the key and ran EXPLAIN on the new query. Saw key in use, and "rows" examined drop from 29,273 to 15.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11490

Differential Revision: https://secure.phabricator.com/D16423