sources: Speed up MySQL ingestion by parallelizing

[def-]

On a cluster with 8 workers, running on my 8 core / 16 thread dev server:

$ bin/mzcompose --find feature-benchmark down && bin/mzcompose --find feature-benchmark --release run default --scenario=MySqlInitialLoadMultiWorker --scale=+1
[...]
NAME                                | TYPE            |      THIS       |      OTHER      |  UNIT  | 'THIS' is
---------------------------------------------------------------------------------------------------------------------------------
MySqlInitialLoadMultiWorker         | wallclock       |           2.042 |          11.416 |   s    |    better:  5.6 times faster
MySqlInitialLoadMultiWorker         | memory_mz       |        1213.754 |        1019.012 |   MB   |    worse:  19.1% more

Co-written with Claude 🤖

[github-actions]

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