Added stats collection

Only sending flows report metrics: send/receive rate and RTT.
I have used an internal interval_start_time instead of env.interval
because the flow also needs to account for its start/end times
when averaging metrics.

I have assumed the same dictionary format as in host.py.

I am not sure what the flow should report if the report function
is called before its start time. Perhaps the report function should
be called on the flow by the main env only after its start time?