make the delay for retry-after static

[gpatel-fr]

so it's kept when the backend is recreated - it happens every time the retry-after is set.
Also, as having the cloud throttling the connection is a serious event, make it a warning instead of a profiling event so it will not be hidden from the user.

[duplicatibot]

This pull request has been mentioned on Duplicati. There might be relevant details there:

https://forum.duplicati.com/t/duplicati-stuck-on-compact-when-getting-putting-from-onedrive-and-results-in-missing-files/16641/53

[Jojo-1000]

I think it might be better to make the retry after variable for each backend that uses it static. That way the different backend APIs are not mixed, although it probably doesn't matter much.

[gpatel-fr]

I don't parse the meaning for 'each backend that uses it'. There is only one backend that uses it: OneDrive. Maybe it was intended to be shared with other drivers but so far it has not happened. That's why I have modified it as if it was under the OneDrive directory, because for all intends and purposes it's the case.

[tygill]

A simple way to make it static per backend would be to have the helper take a ref parameter to track the state in. Each backend that wants to use it could then have its own static that it passes in each time it is called.