Support gathering "top" percentile statistics

Adds support for collecting statistics on top percentiles, instead of
the default bottom percentiles. You specify a top percentile by
using a negative number - so -10 will collect the top 10% of data. It
will emit: mean_top10, lower_top10, and sum_top10.

Using a negative number may seem hacky, but it's convenient and there
is a precedent - referencing an array from the end in some languages
can be done with negative indexes.