AudioWorkletNode lifetime

Hi Group,

I've put together a PR that allows AudioWorkletNodes to control their
lifetime:

https://github.com/WebAudio/web-audio-api/pull/959

After some attempts to do something along the lines of my proposal from the
last call, I gravitated to Hongchan's earlier suggestion that process()
return a value to control the lifetime. However, I had an insight that the
value could affect the existence of a special active reference that
contributes to the node's lifetime, rather than directly sustaining or
ending the node's life. This seems to be a useful idea -- see what you
think.

Best,

.            .       .    .  . ...Joe