set changed status on model to prevent status overwriting when model gets saved

In Cron/Model/Observer the tryLockJob method gets called on the model but the same model is later saved in the main method call. Unfortunately in the memory of the model is still the old status stored so it gets then overwritten and the job is no longer locked and gets triggered multiple times.

This is also a Magento 1 problem, I'm looking forward to a backport, as we have to fix this in every single instance. :(