WebRTC connection of viewer is never closed

[RedMser]

This causes a pileup of connections, which also show in the status API.

• Clean up when leaving the browser page (does not seem to fix the immediate issue, I assume browsers do this automatically anyway?)
• Server-side timeout of a viewer's connection (streamer should not be affected)

[RedMser]

Need to check if this still happens. In solo testing it seemed better now.