Add GAsyncReadyCallback to the introspection API. This makes it possible
to report errors in the future. Don't pass NULL in the callback anymore
now that we have the Ready callback. We also don't need to have a return
value from the callback because now we can cancel any iteration by using
the GCancelable.
Make pinos_context_info_finish() to get the final result/error of
introspection.
Update tools for new API
