The `isc_histosummary_t` functions were written in the early days of
`hg64` and carried over when I brought `hg64` into BIND. They were
intended to be useful for graphing cumulative frequency distributions
and the like, but in practice whatever draws charts is better off with
a raw histogram export. Especially because of the poor performance of
the old functions.
The replacement `isc_histo_quantiles()` function is intended for
providing a few quantile values in BIND's stats channel, when the user
does not want the full histogram. Unlike the old functions, the caller
provides all the query fractions up-front, so that the values can be
found in a single scan instead of a scan per value. The scan is from
larger values to smaller, since larger quantiles are usually more
interesting, so the scan can bail out early.
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