Adjust time with clock drift when validating session

[jogold]

This PR improves the session validation using the computer's clock drift.

It works as follows:

• After authentication, the computer's clock is compared with the JWT iat claim. The difference is saved in the storage.
• When validating the session, the current time is adjusted with the saved clock drift.

(this assumes a constant clock drift between authentications)

I implemented this because we had the case of a user with his computer clock more than 1 hour off in the future (positive drift), the session was never valid.

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This change is 

[jogold]

Hi,

Any chance to get this reviewed/merged?

Thanks