add a statistic of the queue length

I propose the add a statistic about the queue length, exactly the packet queue length in multi-threading mode when a packet is popped from it.

We have the choice between using the Kea statistics library or (IMHO better) to use the similar code than kernels for reporting load averages (which is a strongly related concept) for 1mn, 5mn and 15mn https://en.wikipedia.org/wiki/Load_(computing)