New API surface: symmetric_rtp, nack_retransmit, playout and
playout_max_delay_ms, paced and session.drain(), AdaptivePlayout,
PacedSender, MediaClock, JitterBuffer.mark_non_media and reset.

Behavioral changes: audio sessions no longer keep NACK history by
default; remote SSRC changes relatch immediately; RFC 4733 packets
no longer read as packet loss; IPv6 STUN Binding Responses.