convert para_audiod to the new scheduler.

convert para_audiod to the new scheduler.

As expected, this was a bit harder. It works, but I'm sure there are
plenty of bugs left. Fortnunately, this doesn't matter much right now,
because the next step will be the integration of the writers into the
para_audiod executable. This will lead to major code simplifications,
so let's defer the bug-hunting until this integration is complete.