Currently, the feedback_fn argument to SubmitOpCode is no longer used.
We still need it in burnin, so we re-enable it by making the code call
that function with the msg argument in case feedback_fn is callable. The
patch also modifies burnin to accept the new argument format (msg is not
a triple instead of a string).

The patch also removes the ‘proc’ argument as it's obsolete; instead we
can accept a luxi.Client instance (noone uses this right now).

Reviewed-by: ultrotter