Step-by-step packet debugging

[jonnor]

To investigate errors in the graphs, it would be very useful to be able to step forward in the network execution on a packet-by-packet basis.

This will likely require work in NoFlo and the other runtimes to be able to support it, as well as the FBP protocol to enable.

[djdeath]

Wouldn't it be simpler, in a first time, to be able to replay a recorded set of events, with the graph view explicitly showing where the data flows through. The ability to replay step by step would be helpful to understand what happened.

This would limit modifications to the UI, at least as a first step.