This keeps repo creation serialized (because of a bug in gitea),
but it parallelizes updating the settings. This should reduce
our time by about half.
It also uses a requests session, though I'm not sure if that
really gets us anything.
It eliminates a couple of extraneous GET calls following 302
redirect responses from the POSTs on setting updates.
This will automatically paralellize to nproc * 5 threads.
Change-Id: I5549562d667c0939d0af1151d44b9190774196f9
47bd535d60