Base `JobBatch#complete?` on the presence of `completed_at`.

The old logic was designed to figure out when to mark the
job batch as complete by setting the timestamp -- but
that has been moved into the lua script. Basing `complete?`
on the job jid sets is potentially risky since we have
discussed making some of the job batch redis keys expire
sooner than others. If the job jid set keys vanished from
redis the old logic would wrongly report the job batch
was not complete even though it was. The new logic
relies only on the `meta` key which is the primary key
used by a job batch to track its state.