[collectd] aggregation: CalculateSum sometimes doubles up

[Jake Watkins]

re: https://github.com/collectd/collectd/issues/297

I know this is a known issue but I'm reaching out to see if anyone has
any ideas as to the root cause and/or how to fix this bug.  Not being
able to get reliable data out of the aggregation plugin is a serious
hiccup for our operations and there is talk of moving away from collectd
altogether.  I would much rather fix this issue than find another solution.

-- Jake Watkins
Release Operations Engineer @ Mozilla