Find a better way to align / buffer the audio and control data in adcontrol element

[dholroyd]

If adcontrol applies volume changes to fade the main audio, I presume that the use of InterpolationControlSource should mean a good line up of the volume changes to the audio.

To stop the InterpolationControlSource instance leaking references to all control points ever added, we just delete any points more than 1 second older than the one being added. There's probably nothing that requires the AD_descriptor control data to arrive at the adcontrol element within 1 second of the audio data for the same timestamp (e.g. there could be >1s buffering in a queue-element on one path that's not there in the other).