workload: Create a full fledged flow for listening socket
workload: Create a full fledged flow for listening socket

While it does not apply to TCP workloads, for UDP workloads the
"listening" socket is also the receiving socket so we need to create
a flow for it sample the number of received bytes at regular
intervals.

The flow tear down is not as clean as it could be. We needlessly close
the socket twice.

jsitnicki/rushit,jsitnicki/rushit