Fix CardinalityViolation in grab_next_visits on duplicate origins

grab_next_visits grabs from listed_origins, whose primary key is
(lister_id, url, visit_type) and uses it to upsert in origin_visit_stats,
whose primary key is (url, visit_type).
This causes the error `ON CONFLICT DO UPDATE command cannot affect row a
second time` when the same (origin, type) pair is grabbed twice.

This commit deduplicates the (origin, type) pairs before upserting.