It's fairly silly to truncate the throttle_delay to integer when the only
math we ever do with it requires converting back to double.  Furthermore,
given that people are starting to complain about restrictions like only
supporting 1K client connections, I don't think we're very far away from
situations where the precision loss matters.  As the code stood, for
example, there's no difference between --rate 100001 and --rate 111111;
both get converted to throttle_delay = 9.  Somebody trying to run 100
threads and have each one dispatch around 1K TPS would find this lack of
precision rather surprising, especially since the required per-thread
delays are around 1ms, well within the timing precision of modern systems.