Race condition in test_reaching_max_in_flight_requests

[casey]

I think I found a race condition in test_reaching_max_in_flight_requests which causes this assertion:

assert_eq!(32, c1.load(Ordering::SeqCst));

to fail because the rhs is equal to 33. I noticed it organically a couple of times while running the tests. After experimenting a little bit, I found it is hit every time if you add a 1 second sleep before that assertion, as in this commit:

casey@99f54ed