SERVER-10026 tighten index bounds for some array equality queries

The bounds for a query like {a: [1, 2, 3]} in the new query framework are currently [MinKey, MaxKey]. This change optimizes array equality queries by tightening the index bounds. The bounds for the query {a: [1, 2, 3]} should now consist of the two point intervals [1, 1] and [[1,2,3], [1,2,3]].