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.