Improve precision of slot_slide histograms

If traffic is bursty, precision can suffer in the slot_slide
histograms, especially during test runs when the runs are short
(e.g. all values enter into the same slot, leaving only a few
values for calculating the stats). To help this, the slot_slide
function keeps a subset of the values around and includes them
in order to get a sufficient number of values for the analysis.
The logic is very blunt right now, and no attempt is made to
improve the spread. The data set is padded up to 300 values right
now. The min, max and mean values are accurate, but percentiles
may be a bit off - roughly as much as folsom's slide_uniform
implemementation.