Make next_visit_queue_position an integer
In visit types with small amounts of origins having no last_update field, we would end up overflowing Python datetimes (which only go up to 31 December 9999) pretty quickly. Making the queue position a 64-bit integer should give us some more leeway.
The queue position now defaults to zero instead of an arbitrary point in time. Queue offsets are still commensurate with seconds, but that's mostly to give them some space to be splayed by the fudge factors.
Related to #3748 (closed) Sentry report https://sentry.softwareheritage.org/share/issue/fa61b8333b0c4c429f8cf1356e3fc08d/
Test Plan
- unit tests updated with the new dimension for the queue position
- tested the migration script locally
Migrated from D6777 (view on Phabricator)