Locally, I witnessed very slow tests. Profiling with Visual VM
revealed a lot of time spent in `wait`.
Digging in, we were trying to be clever, with a `wait(1000)/notify`
mechanism. However, there were never multiple threads in play, so the
waiter wasn't waiting when `notify` was invoked. That means we always
timed out. I think this never worked and using bare `wait()` would
have revealed that.
Anyway, `CountDownLatch` maintains the one bit of state (was I
notified) and generalizes smoothly to when we have threads.
