Reduce memory usage during startup

In the case of large backlogs of persistent messages (10s of millions
of messages)

Previously we queued a job for every file with worker_pool:submit_async,
however if there are 50 million messages, this corresponds to ~79,000 files
and the same number of pending tasks in the worker pool. The mailbox for
worker_pool explodes under these circumstances, using massive amounts of
memory.

The following was helpful in zeroing in on the problem:
https://elixirforum.com/t/extremely-high-memory-usage-in-genservers/4035

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