Add more calculations on timer/latency values

I'm not really sure about the last commit in the set. Is it ok to submit values of type `size_t` ? And what about overiding the `vl.type` on line 782 (cf comment inline) ?

I have a batch of comments I plan to post on https://github.com/collectd/collectd/pull/358. This patchset may make more sense after that.