Prevent RevalidateCachedPlan from making any permanent change in

Prevent RevalidateCachedPlan from making any permanent change in
ActiveSnapshot.  Having it affect ActiveSnapshot only in the unusual
case of needing to replan seems a bad idea, and there's also the problem
that the created snap might be in a relatively short-lived context, as
noted by Jan Wieck.  Also, there's no need to force a new snap at all
unless we are called with no snap currently set, which is an unusual
case in itself.