Currently the order of extraction can vary based on which ABC runs finish first. That's
nondeterministic, therefore bad. Instead, force the processing to happen in the same order
as `assigned_cells`, i.e. the same order we use when not using parallelism. This should
make everything deterministic.
Note that we still allow ABC runs to complete out of order. Out-of-order results are
just not extracted until all the previous runs have completed and their results
extracted.
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