PostgreSqlSliceStore: quicken segment retrieval

When a segment was associated to more than one block in the request,
multiple rows were being retrieved. Although these segments are
eventually removed by hash collision and so do not affect results, there
is still a performance penalty to querying and parsing them. Instead
don't group by block, so that each segment has only one row, and let the
database prevent duplication of these segments' slices. Increases
segment retrieval throughput by 50%.