limit collate task to 50,000 messages
limit collate task to 50,000 messages

we've seen issues where tasks mysteriously hang and do not process
large volumes of letters - in this case >150k letters in created state.

to try and get at least some letters out of the door, limit the query to
only return 50k letters per postage type. We may need to run the task
multiple times, or letters may get delayed until the next day when
they'd be picked up (provided there's enough capacity then). The task
should only be re-run AFTER the ftp tasks have all finished, and updated
the letters to sending, or we run the risk of sending the same letters
twice.

For context, the largest ever letter day we've sent is ~65k in march of
this year.

alphagov/notifications-api,alphagov/notifications-api