backlog, with queue.max_oldest_due_ms set: the oldest due delivery row has
waited no longer than that (the figure ankusa_queue_oldest_due_age_seconds
reports, from the same read). Over it the reason is over_threshold.
A label, never a reason to be unready: a node with a backlog still takes
hooks, and taking it out of the load balancer would only move the load. A
store that does not answer is unavailable (the store check is what
gates).