burnin: move batch init/commit into a decorator

burnin: move batch init/commit into a decorator

Many burnin steps initialize the batch queue at the beginning and commit
it at the end of their operation. This patch moves this code to a
decorator, in order to reduce redundant code.

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