Codec negotiation for multi-party conferencing
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RTP multi-party conferencing requires that all parties use the same list of codecs. There are three ways to ensure that.

- Have a static pre-determined list of codecs and use it on all
  clients

- Negotiate a common list in a centralised manner

- Negotiated the common list in a decentralized manner

Farstream supports the first two techniques, but not the third (which
is a lot more complicated and error-prone).

To use technique one, one just needs to call
fs_stream_set_remote_codecs() with the same list of codecs on all
streams (actually, on at least one stream per session).

To use the second technique, one must designate a Focus, which will
generate the list that every one node will use.

The non-focus nodes will send their list of codecs to the Focus and
accept whatever the focus sends them as a reply (and use that).

The focus node will first receive a list of codecs from a node, and it
will perform the normal negotiation (prefering the remote payload
type), but when a second FsStream is added to the FsSession, the
behavior of the codec negotiation changes and the only effect of
setting remote codecs from other nodes will be to reduce the possible
codecs to the common set (ie the intersection). If there is no
intersection between the current pool of codecs (from the negotiation
with the previous nodes) and the codecs added by a new participant in
the session, this participant will be rejected and
_set_remote_codecs() will fail.