Don't assume mono-audio in defaults
Fit ratox to new toxav-API

mannol has done a great job re-working the toxav-api[0] also based
on our suggestions for a stricter callback-approach.
Previously, toxav-callbacks could hit any time, leaving us with
dramatic timing-issues and instability in certain situations.
The new toxav_do()-call enables the client to control when to
fire the callbacks. toxav_do_interval() makes sure no time is
lost in the meantime.

[0]: https://github.com/irungentoo/toxcore/pull/1163