The value returned by histogram_percentile() is an upper bound estimate.
This is fine for measuring latency, because we are interested in the
worst, i.e. highest, observations, but doesn't suit particularly well
if we want to keep track of the lowest observations, as it is the case
with bandwidth. So this patch introduces histogram_percentile_lower(),
a function that is similar to histogram_percentile(), but returns a
lower bound estimate of the requested percentile.