* Add index on webhook_deliveries.created_at
The stale scope (WHERE created_at < ?) and ordered scope
(ORDER BY created_at DESC) both benefit from this index.
Without it, Webhook::Delivery.cleanup does a full table scan.
* Run webhook delivery cleanup every 15 minutes
Each run now deletes ~15 minutes of newly-stale records instead
of ~4 hours worth, a 16x reduction in per-run write volume.
Jeremy Daer
bc76633f77