Add a way to limit bandwidth for avatar and audio mixer

[ksuprynowicz]

On some internet connections built in congestion detection doesn't work well, causing packet loss. A client setting to manually limit incoming and outgoing stream bitrates from avatar and audio mixers would be very useful for this. It would be also very useful for connections with data caps, for example mobile connections.

[daleglass]

There's #545 which improves audio bandwidth setting