For external indices, we'll need to support nested cat_async
invocations to deduplicate cross-posted messages.
Thus we need to ensure we do not clobber the {inflight*} queues
while stepping through and ensure {cat_rbuf} is stored before
invoking callbacks.
This fixes the ->cat_async-only case, but does not yet
account for the mix of ->check_async interspersed with
->cat_async calls, yet. More work will be needed on that
front at a later date.