Sending NACK/FEC is not supported

[Philipel-WebRTC]

Given that each RtpSendStream bundles both the media and RTX SSRC into one object it is currently not possible to send a packet on the RTX SSRC.

Some ways to fix this:

• Unbundle media and RTX SSRC as suggested in Remove associated rtxSsrc from RtpSendStream and RtpReceiveStream. #50.
• Make SSRC modifiable by the app, then RtpSendStream.sendRtp would know which SSRC to send it on.
• Add a sendRtx function.

I also believe bundling media and RTX SSRC makes things such as flexfec (FEC protecting multiple media SSRCs) slightly akward as you would have to just pick some SSRC to send it on.

[steely-glint]

Place holder for the 'use case 3' doc -: We'd like to be able to manage the packet cache used by NACK, so that for example older NACKs will be ignored. - I don't much care how it is done ;-)