Multi-hop latency measurement example

[eboasson]

This adds a simple tool for doing latency measurements across multiple hops. It assumes the clocks are synchronised to a high degree so that one-way latencies can be computed directly.

It can operate with a number of different types, all very simple:

struct Hop8 {
  uint32 seq;
  octet z[8 - 4];
};

and variants where the total size is 128, 1k, 8k and 128k bytes. Each process takes a stage, with the source publishing in partition Pj, the sink subscribing in partition Pj and the forwarders subscribing in Pj and publishing in Pk, where j is the stage argument and k = j+1.

Each process additionally subscribes to "junk data" and optionally publishes samples at randomised intervals with a configurable average rate.