Don't use 1.5 GB when importing a big maildir.

The code was creating 70k ItemCreateJobs first, and then starting them all.
That makes mem usage reach a peak of 1.5GB here.
It does have code to do one event loop iteration every 20 e-mails,
but that only makes it fire one ItemCreateJob, so they pile up anyway.

Solution is to throttle ItemCreationJob creation.

CPU-wise, this solution and the old one are equivalent,
they are both equally fast.

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