Instead of using the full sample as statistics providing
enough information to compute the standard deviation, use
a slightly more elaborate one. It contains the standard
statistics count, sum, and sum of squares, which can also
very easily be updated. However, computing the standard
deviation from these values unfortunately is not precise
enough for our purpose (small difference of large values).
Therefore, we also carry the naively computed variance in
the statistics. The latter can also be updated using the
standard statistics and, moreover, this can be done in a
way that does not introduce too large inaccuracies for a
small number of updates.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Petr Pudlak <pudlak@google.com>