Add missing indexes to DrydockRepositoryOperation
Summary: See PHI1885. Repository operations are queryable by state and author, but neither column has a usable key. Add usable keys.
Test Plan: Ran EXPLAIN on a state query. Ran bin/storage upgrade. Ran EXPLAIN again, saw query go from a table scan to a const key lookup.
Differential Revision: https://secure.phabricator.com/D21465