sync: switch network fetch to multiprocessing

This avoids GIL limitations with using threads for parallel processing.

This reworks the fetch logic to return results for processing in the
main thread instead of leaving every thread to do its own processing.

We have to tweak the chunking logic a little here because multiprocessing
favors batching over returning immediate results when using a larger value
for chunksize.  When a single job can be quite slow, this tradeoff is not
good UX.

Bug: https://crbug.com/gerrit/12389
Change-Id: I0f0512d15ad7332d1eb28aff52c29d378acc9e1d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/298642


Reviewed-by: Chris Mcdonald <cjmcdonald@google.com>
Tested-by: Mike Frysinger <vapier@google.com>