The time that a message between PNF/VNF to arrive depends mostly on the
transport. To allow for some delays, there is a slot_ahead time, during
which the VNF is allowed to schedule and send instructions to the PNF.

This can be multiple slots; the 1ms hitherto given might typically too
short. Increase to 10ms, to encompass a wider range of slot_ahead times.

Make the corresponding log message of when old (stale) message are
removed a bit clearer with respect to times.

This is the first commit in which nFAPI works. Follow-up commits improve
performance.