RTCMultiConnection.leave calls stop method on the streams (and other fixes)

In my version, the method `connection.leave` also calls `stop` on the open streams; this means that after calling `connection.leave` the webcam goes off without a page reload, thus shutting down the webcam LED and lowering CPU usage. Subsequent connections require new RTCMultiConnection objects.

Moreover, (I think) I fixed a bug that prevented the multi-chat to work correctly using v1.3.