Make the device switch on audiounit cheaper. Right now we destroy and recreate everything. We can just stop the audiounit, assign the new device id and start it. This will likely improve the device switching experience. For now, it involves a little pause in audio and video.

The following branch contains a prototype of the solution: https://github.com/achronop/cubeb/tree/osx-improve-reinit It does not work well with aggregate devices. I will stop working on this for now since other priorities have arise.