[webrtc-pc] should RTCRtpSender.setParameters invalidate LastReturnedParameters?

fippo has just created a new issue for https://github.com/w3c/webrtc-pc:

== should RTCRtpSender.setParameters invalidate LastReturnedParameters? ==
from discussion between me and @Orphis:

I assume we want getParameters/setParameters to be used like this:
```
const sender = pc.getSenders()[0];
const parameters = sender.getParameters();
parameters.encodings[0].maxBitrate = 500000;
sender.setParameters(parameters);
```

but in Chrome's current implementation (very new; still behind flag) one can use those parameters multiple times which afaics matches the spec.
I assume we do not want this:
```
const sender = pc.getSenders()[0];
const parameters = sender.getParameters();
parameters.encodings[0].maxBitrate = 500000;
sender.setParameters(parameters)
.then(() => {
  // fiddle with something else in parameters;
  sender.setParameters(parameters);
});
```
as we would have to determine whether the modification conflicts.
The easiest way to achieve this seems to have setParameters clear LastReturnedParameters.

Test-as-you-commit:
eeds a test showing that the second bad example fails.

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/1857 using your GitHub account