With futures 0.3, we can check whether a sink can accept items without actually
sending them to it, which used to be the reason why we needed to per-destination
buffers.
We've been passing around `Stream`s of `Frame`s everywhere even though we never
made use of the async nature of those (i.e. we used future::ready everywhere).
This resulted in unnecessarily complicated an hard to read code. Instead we now
have a simple VecDeque outbound_buf which we push packets on if we want them to
be sent. No more passing around complicated return values.
Delayed packets weren't handled properly and when a transmission timed
out, RTP sequence numbers were re-used and therefore rejected by the SRTP layer.
