Fix fileserver backends global overwritten by _pillar

[bersace]

Refs #22941

[thatch45]

Woah! This will make all masters run 20 times slower!
Lets take a closer look at where the underlying issue is

[cachedout]

Agreed with @thatch45 here. This will cause a massive performance hit to the master. I am going to close this PR in favour of continuing this discussing in #22941. Thanks, @bersace

[bersace]

Hi,

I also feel uncomfortable with this solution. But at least, it's a starting point to fix this :)

The ground of the issue is : fileserver backends are singleton with __opts__ pushed in the context of the module. Pillar monkey patch the backends by reloading file client with pillar=True. So we must a least restore.

The real solution would be to refactor fileserver backends as class with a context. I think self.opts is enough for a context ;-) But for now, the fix can be to just clean the monkey patching after pillar are loaded.

This remember the ages of guerilla patching in Zope ;-)

Thanks for reviewing this :)