multistream: make the daemon able to negotiate more than 2 medias

This patch makes the daemon able to negotiate more than 2 medias
for a call. Basically, this patch includes some major changes:

1. reportMediaNegotiationStatus is also called for conference, so
the client is able to update the mediaList of the conference to
use it later if it needs to change the medias. And the medias
can be retrieven with currentMediaList
2. The conference object correctly updates the video mixer on
media changes, and video inputs are restarted if removed then
re-added.
3. Because previous versions of the daemon only supports two medias,
only negotiate more medias with compatible peers. If the daemon is
not compatible, the medias will not be negotiated anyway.

jami-project#1429

Change-Id: I55701d9e6e53b05c9a58adad4e8ea90303e6b9e7