burnin: move decorators out of classes

Similar to commit c881c5, we move the decorators out of classes, such
that they become simple functions instead of methods. This more clean,
since only the wrapped functions need to be methods/have access to
‘self’.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Olivier Tharan <olive@google.com>