burnin.ExecOrQueue: add post-process function

If a post-process function is passed to ExecOrQueue it is executed if
and only if the job is successful. This happens immediately if we're
proceding iteratively, and at the end, when we collect all job results,
if we're proceding in parallel.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>