Fix Sink API usage in P2PStream

[Rjected]

Ref #114, currently the stream uses send from SinkExt in multiple places, which completes when the Sink has fully processed the item. Instead, it should be using the Sink API like this, buffering requests when the Sink is not ready:

if poll_ready().is_ready() {
   start_send(msg)
} else {
  // need to buffer until sink ready.
}

[mattsse]

we could set some sensible default capacity for the sink buffer.