The opposite of 'EXP_Ttl(req, oc) > req->t_req' should not have been
'EXP_Ttl(NULL, oc) < req->t_req'. If we somehow enter the lookup when
the two operands are equal, the objcore suffers a phenomenon known as
Schrödinger's expiry.

The chances of running into this scenario range from epsilon to 100%.

Because 't_req' is stable across restarts, a soft purge will reliably
trigger this case.

Test case by Alve Elde who first demonstrated the problem.