How many times a single ordered consensus item is processed before we give
up. Only a database transaction that could not be validated at commit time
costs an attempt, and reprocessing takes a fresh snapshot, so needing more
than a couple of these means something is wrong beyond a stale snapshot.
The request of a shutdown that takes effect now that completed_session
has completed: at the first session boundary at or after the scheduled
session, so a schedule that arrives after its session completed still
stops at the next boundary rather than never.