Use 2 channels for the OPUS_RTP format (the payload-rtp-opus draft requires
opus to be advertised with 2 channels)
Increases the default priority of SILK, making it prefered over G722.
Removes stereo formats from the opus encoder's supported input formats, because
problems were observed when they were used (and we don't yet support stereo
decoding anyway).
Prevents the two-channel opus format from bumping MAX_AUDIO_CHANNELS to 2.