visit_stats: Keep a reference to the last snapshot

To allow more fine-grained deducation on the eventful/uneventful nature of last visits

Related to #2967 (closed) Depends on !170 (closed)

Test Plan

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

• [1] swh-model!151 (closed)

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Migrated from D4853 (view on Phabricator)