`sleep_async` behaves as if it's blocking, strange

[PMunch]

You don't need `--threads:on` for Jester, in fact it doesn't do anything. And 
these won't be executed in parallel, but they will be executed concurrently. I 
have also ran your snippet and do in fact get the "start, start, finish, 
finish" result that is expected, without the `--threads:on` flag.