Shorten bitrate calculation window to adapt for bandwidth changes faster

[dsmeytis]

This PR is related to PR#1439. It would be better to reduce bitrate calculator back window in order to speed up RateStatistics warm-up period and reach minimal bitrate threshold faster.

[jitsi-jenkins]

Hi, thanks for your contribution!
If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.

[bgrozev]

@dsmeytis Thank you for the contribution and sorry for the slow response. I don't feel comfortable reducing the window size this much without testing, but #305 will allow you to easily configure it for your environment. Does that work for you?

[dsmeytis]

Hi, @bgrozev. Yes, that's good opportunity to configure window. So this PR is obsolete now. thank you.