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While all methods on tasks are called within the manager's lock,
dispatching to another queue obviously means the manager may not
be locked anymore.
I also moved calling progress download callbacks to the download
task so it's similar to progress image.
Hopefully these issues have been fixed, but it leads me to believe
a better architecture would be to make the tasks immutable by the
manager and have the tasks manage a lock on themselves. I'll need
to think about that more.