videoroom: enhanced multistream support

[atoppi]

This PR tracks the efforts aimed at integrating multistream APIs in the videoroom. This patch is still a draft do tue some missing APIs and lack of testing, but the demo should work.

If you are interested in vr multistream in janode, please test and report. I'm also interested in discovering use cases still not covered by the current status of the implementation.

Missing/untested APIs, TODO:

• [x] update with unsubscribe
• [x] switch
• [x] rtp forwarding
• [x] simulcasting
• [x] talking events

[atoppi]

We've implemented and tested most of the APIs. Removing the Draft status now, hoping for some feedback :-)