Upload large files (the ones that go through the resumable upload path) with a single thread.
Upload large files (the ones that go through the resumable upload path) with a single thread.

This seems to help with the flakiness and large number of rate limit
errors that come up with doing that concurrently with regular uploads
of smaller files.

pnu/skicka,clonemeagain/skicka,google/skicka