`searchNotificationAvailable` never effectively use any indexes because:
- `notified_at < $1`, where $1 is a recent timestamp, returns the
majority of the table and therefore it is cheaper for PostgreSQL
to use a sequential scan on the table.
- there is no index for `deleted_at IS NULL`.
However, when Clair has been running for long enough, the grand majority
of rows (99%+) are expected to have a non-NULL `deleted_at` field. This
commit adds a new index on this very field in order to fetch the
remaining 1% in the blink of an eye.
In other words, instead of realizing a full table scan for each
`searchNotificationAvailable` query, we'll use the small branch of a new
index, reducing the total cost from over 30k to a mere 150 on a Clair
database that has already managed more than 1 000 000 notifications.
