Revert I4afaecd8: Avoiding writing sessions for no reason
I4afaecd8 reduced the frequency at which the TTL of sessions keys is reset, on
the assumption that we were doing it a whole lot more often than necessary.
We can now assume that this assumption was false, because an uncommitted revert
of this change which I pushed to production earlier caused the rate of session
loss errors in the logs to plummet.