model: use visit ids in the unique key, instead of their date.
dates are not unique (ie. multiple visits can share a date, and they do in practice) and timestamps are also not great for testing equality (eg. migrating between systems is lossy). Additionally, visit statuses already use visit ids in their unique key.
Depends on !139 (closed)
Resolves swh-journal#2672 (closed)
Migrated from D4194 (view on Phabricator)