write better version-uptake measurement tool

[warner]

I've got a rudimentary munin plugin that takes the client-version data from #3 and attempts to make an area chart of how many clients were using which versions. My intention was to sum the usage numbers in a rolling window, spread over the last hour or so. But I did it wrong, or it's interacting with the blurred timestamps in the logs somehow, and I wind up with weirdly-truncated graphs like this one:

The lack of data for the last 12 hours is issue #13, and unrelated to this one. This bug is that the graph grows from zero at the start of each hour, to some peak, then resets back to zero again.

I suppose having all the timestamps rounded down to the nearest 60 minutes might explain this, in which case I should really be summing over the last 12 hours or so.

[warner]

Yeah, summing over 12 hours makes it better. There's still a slope within each hour, as new connections get accumulated (all of which are blurred to the start of the hour), but it's more legible now.

The next-more-smooth approach I can think of is to sum over a 12-hour window that finishes an hour ago (i.e. from T-13h to T-1h), rather than the 12 hours finishing now (T-12h to T-0h). That would remove the slope, and give us something shaped like a bar graph.