The snapshot is used to determine the "eventful/uneventful" nature of the origin visit
status.
When no snapshot is provided, the visit is considered as failed so the last_failed
column is updated.
As there is no time guarantee when reading message from the topic, the code tries to
keep the data in the most timely ordered as possible. Only most recent information is
kept.
TODO: @vlorentz mentioned in D4852, api origin_visit_stats_upsert should be batched.
Related to T2967