Proposal for RtpSender.getParameters and .setParameters

At TPAC (2014), we reached consensus that we want some form or
RtpSender.getParameters and RtpSender.setParameters.  But we had three
issues to resolve:

1. What's the async pattern?
2. What's the current vs. proposed?  (punt;  when we deal with
getProposedLocalDescription)
3. What's the minimal set of controls?

For #1, I propose we return a Promise from setParameters(), like so:

partial interface RTCRtpSender {
  RTCRtpParameters getParameters();
  Promise setParameters(RTCRtpParameters parameters);
};

If setParameters fails, the returned Promise fires an error.  If
setParameters succeeds (and when onnegotiationneeded is fired for
parameters that require negotiation), then the returned Promise fires
success.

.getParameters() always return the current parameters being used to send.

For #3, I propose the following as the absolute minimum:

dictionary RTCRtpParameters {
  sequence<RTCRtpEncodingParameters> encodings;
}

dictionary RTCRtpEncodingParameters {
   boolean active = true; // sending or "paused/onhold"
};


I think everyone agrees that we would want that in our minimal set.  And to
that minimal set, I also propose we also add maxBitrate:

dictionary RTCRtpEncodingParameters {
    boolean active = true; // sending or "paused/onhold"
    unsigned long maxBitrate = null;
};

But please propose your own additions to this minimal foundation.