av refactor

Summary:

  - All the toxav work is now handled in threads provided by the user
  - Using toxav_do_interval as suggested by @FRIGN in #1156 
  - cleaned up code
  - changed av callback behavior
      - onStarting and onStarted is now merged into onStart which is called when call is started
      - onEnding is merged with onEnd which is called when call is ended
      - onMediaChange is split into onPeerCSChange (peer requested cs change) and onSelfCSChange (cs change confirmation)
  - removed horrible comments etc...