Fix the calculation of rates in ConnectionStatistics

[lachlan-roberts]

closes #5783

The calculation for the rate was previously done as getTotal() / elapsedTime.
When it should really be getIncreaseOverElapsedTime() / elapsedTime.

[lachlan-roberts]

@sbordet another problem with this is that ConnectionStatistics doesn't collect live statistics from connections. It only updates when the connection closes, so its probably not going to give you great statistics for rates in websocket. You'll probably be using long lasting websocket connections so the rate values would probably be pretty useless.

[gregw]

Maybe have some kind of 'takeStats' method that updates all the totals and remembers the levels in each connection so it doesn't get double counted.
call takeStats only if somebody queries the stats... not sure if all the wiring necessary is there... but look

[gregw]

I'm +0 as I don't really like the total being included in the RateCounter.
I can see how it makes the ConnectionStatistics a little neater, but it is kind of unrelated.