Compute load based on audio, video and screenshare streams

[pielonet]

Hi,

I propose this pull request based on this one #445
I add the enhancement that load is actually computed based on actual audio and video streams.
We add up 3 values :

• Normal audio users count for 1
• Webcam users count for 10 (a webcam stream is roughly equivalent to 10 audio streams in terms of traffic)
• A screenshare counts for 20 (a sharescreen stream is roughly equivalent to 2 webcam streams in terms of traffic)

These values are multiplied by the actual number of participants to get the actual number of streams treated by the server (SFU principle).

[einhirn]

Perhaps this can be combined with #476 - this uses a configurable base load for the whole time (just reverts to actual user count after a configurable time), the PR I created with @defnull's code computes the actual count of video and audio streams.