Add a -slow-start flag to ramp up requests

[klingerf]

When sending a lot of load at a target, it would be nice to have slow_cooker ramp up to it's total qps over the course of a minute or two, rather than just sending the full amount of load at the outset. This would give the target a chance to warmup.

[stevej]

Yeah, a warm-up phase is pretty critical for services that rely on GC or a good cache hit rate.