Add tool to determine when packets dropped

[wingo]

The ptree black-box facility stores historical counter change rates. Sometimes we know that packets have dropped, e.g. by looking at the raw counters, but we want to know the times at which they dropped. It's irritating to have to scrub through every second with the "snabb top" tool to find this out. So, it would be useful to write a tool to traverse the RRD files and find interesting things for us.