Underlying problem in Wrk: percentile distribution is sometimes broken

[chriskilding]

Sometimes a chunk of the response time percentile distribution is all zeroes, missing data.

This is not a bug in this driver, instead it is almost definitely a bug in Wrk itself.

There's no clear set of input parameters that cause this, and the amount of the distribution that's missing varies between test runs when this bug chooses to exhibit itself.

Reported to the Wrk project at wg/wrk#354