Finally got checksum verification running in a separate job pool from
downloads, to better keep bandwidth saturated.

I had to resort to what felt like a bit of a hack, but I can't see a better
way to do it. Also, I got stuck for far too long on a STM deadlock bug.

Interestingly, this means that -J1 now has a purpose, it's not the same as
no -J option. Instead, it lets one download and also one concurrent
checksum of the previous download run at the same time.

It would be nice if -J1 could be the default.. One problem with that is
that it needs a unicode locale to work due to a limitation of
concurrent-output. Changing the concurrency method based on the locale does
not seem like a good idea.