Add Tx/Rx rates to stream statistics

[pstavirs]

The requirement can be broken down into -

• Calculate and show per stream average rates as TotalPkts/Duration - non real-time

This is doable. But requires tracking of transmit/receive durations. Tracking receive duration specifically is also a UI/UX problem (how does the receive port know when the transmit port has started and finished transmission?).

Update: #350 implements this - it uses only the Tx duration for calculating rates

• Make port rates children of total row instead of additional columns

Essentially convert the table to a 2-level tree - this is easier to read. Example below

SGUID	Tx Pkts	Rx Pkts	Pkt Loss	Duration	Avg Tx Rate	Avg Rx Rate
101 (Total)	...	...	...	...	...	...
101 (Port0-1)	...	...	...	...	...	...
101 (Port0-2)	...	...	...	...	...	...

• Real time stream statistics/rates

This has technical challenges from a performance point of view for both Tx and Rx. Real time tracking is CPU intensive which means CPU used for actual packet Tx will come down, impacting max Tx rates - not sure if this should be done.

• Real time graph of stream rates

This should be doable, but has the previous item as pre-requisite. Also, we should probably implement real time graph first with port stats before this item.