#72 AudioPlaybackRt: On construction open stream, keep open until destruction

[Nick]

When an AudioPlaybackRt object is created using JACK, the JACK stream can be recorded until it's stopped, once it's restarted, JACK clients can't record from it because the stream has changed.

To do:

• Upon construction, start the portaudio stream playing silence
• Upon "stoppoing", clear the buffers with silence, keep stream playing silence
• Upon destruction, stop the portaudio stream and close it