better handling of intervals

* reducing the actual processing time from the interval, so it runs
  in more accurate intervals independent of the processing time.
* if processing time takes longer than the interval, we run it instantly
performance - reducing carbon footprint :)

* Establishing one carbon connection for all plugins, instead of one connection
  for each. This rather drastically improves performance. From a few measurements
  this reduces processing time by over 50%
* This can be even further enhanced, to keep one persistent carbon (/munin?)
  connection and only re-establish it if it drops (For Future)