alfred: add statistics (uptime+traffic)
alfred: make traffic counters absolute

After a lenghty discussion, we settled on absolute vs. relative values.
Main reasons:

* stateless implementation on node possible
* convertable to relative values by differentiaion on receiver
* missed transmissions only decrease granularity, whereas relative
  values would introduce wrong numbers on integration if values are
  missed
alfred: add random node-specific delay before sending
alfred: clean up delay code