Concurrency fixes

[mgeisler]

Hi folks! I'm teaching the Concurrency class, so I went through it and made a bunch of changes.

I debated trying to split this up into smaller PRs — some of the commits here depend on the previous commits, so it's a bit annoying to split up.

If you're okay with looking at this via the individual commits, then it's probably easier to merge it like that. Otherwise I can try splitting things up after teaching my class tomorrow.