Create generic session callback handler.

Current version simply handles cases where only the session is passed in.
Future versions will likely use va_list to support cases like music_delivery.
Switch play token lost to generic session_callback helper.
Switch meta data lost to generic session_callback helper.
Ensure search callback handles memory correctly.
Switch notify main thread to generic session_callback helper.
Add support for extra object into session_callback
Switch all session callbacks except delivery to helpers.
Cleanup module init code a bit adding todo for further debuging.