visit_stats: Update references when per event date is more recent

Prior to this commit, the upsert did not take into account the dates and systematically overwrote previous "most recent" inputs.

This now tries to update only when the provided date (per event) are most recent (events: eventful, uneventful, failed).

The last_snapshot entry is then only updated when the last_eventful date is most recent then the one currently stored.

Related to #2967 (closed) Depends on !171 (closed)

Test Plan

tox (failing until swh.model > 0.9 [1] is released)

• [1] swh-model!151 (closed)

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Migrated from D4859 (view on Phabricator)