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

[prymitive]

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.

[prymitive]

I wanted to cleanup my open issues - is this one mergeable? I'm not sure if this is really needed but it might be useful.

[prymitive]

This could be useful for plugins like distributed crons - if we now we are stopping we should stop executing tasks

[unbit]

i am not if it is still an issue (5 months elapsed). We moved the carbon plugin to the "stats pusher api"

[prymitive]

let's close it, there are not obvious benefits