postgresql: ensure origin_visit(_status) queries use index

When using an inner join for the single origin value, instead of a
subquery, the query fails to use the (origin, visit) indexes and falls
back to fetching all the visits (or all the statuses) for the origin and
sorting them.

This breaks down for origins with a lot of visits, such as the ones that
are being used for end to end monitoring.

Using a subselect to generate a single origin id value ensures that the
queries can use the proper indexes.