Add origin_visit API entry points to permit creation/update

The swh.storage.storage API should permit to directly create and update origin_visit entries. Thus permitting to register one's visit's state.

Suggested API:

• origin_visit_add(origin, ts): id
• origin_visit_update(origin, origin_visit_id, completion): None

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Migrated from T538 (view on Phabricator)