We now ensure that all message ids are sorted BEFORE
we split them into batches.
We now do a few extra "slim" queries to get message
ids up front.
But, now, when we divide them into batches, we no
longer run 2 or 3 different complicated queries in
a loop. We just basically hydrate our message ids,
so `write_message_partials` should be easy to reason
about.
This change also means that for tiny realms with
< 1000 messages you will always have just one
json file, since we aggregate the ids from the
queries before batching.
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