stop calling each plugins master_cycle() when uWSGI is stopping or reloading

I'm not sure if this is needed but if calling master_cycle() in plugin is blocking master, than we should disable those calls once we know uWSGI is shutting down. This way we can have faster stop/reload - or at least we minimize the risk of hanging there (if there is any).
It looks like to_hell means shutting down and to_heaven means reloading so I used those.
For Your consideration.