Guesstimate partition boundaries from extrema rather than using expensive offsets

Summary:
Using order by and offset makes the partitioning a n^2 operation on the number
of entries in the table, rather than an instant operation when using
min/max.

This assumes the indexable column is more or less uniform, which is not exactly
true but not the worst approximation either.

Test Plan: tox

Reviewers: Reviewers, douardda

Reviewed By: Reviewers, douardda

Subscribers: douardda, swh-public-ci

Differential Revision: https://forge.softwareheritage.org/D1267