Configure video stream using negotiated media description

Handle dial out conference while there is already an ongoing conference:
- the conference server will always initiate call in dial out conferences if there is no one yet established
- clients that are already in a conference call will reject any new incoming call from the same conference server with a 486 Busy Here

If a participant requests a video stream of another participant whose video direction is inactive or recvonly, then the conference server will reply with an inactive stream
Remove label and ssrc from participants whose video is disabled
Do not send separate notifys for changes of an ssrc and media capability or availability changes