Round up estimated-dataset-size

[pond]

PostgreSQL's fast count estimation tends to be very inaccurate a lot of the time, but if we assume it might be at least of some vague use on large dataset sizes, we should at least do something for small ones - if the estimated size is less than the actual list count of a returned chunk of data, Hoodoo ought to rewrite it to match the actual known size.

Low priority / nice-to-have.