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Sessions should expire when the time between session:DeactivateSession and session:ActivateSession is more than X minutes.
Not when the time between ActivateSession and DeactivateSession is too much, that would be just bad performance. It turns out that imo is checking that too, currently.

So the only time you need to look at expirationtime is during ActivateSession, and the only time you need to reset it is during DeactivateSession.

Instead, the expiration is now checked and reset during every locateContextValue() in ContextDatabase.cls. That's a lot of times in an active session, while only the first check and the last reset are relevant. That causes several problems:
- waste of time
- lock on ContextSet record (for add-interval) has bugged us once or twice

and the most important one: not every context is a sessioncontext.
You can have context for ever kind of process, session is just one example. A statefree server cannot control if clients have abandoned the system without saying goodbye, so a timeout is a valid guard. But for other contexts it is not. So the expiration should be checked and reset by the Session class, which can still use the Context class to store and fetch that.