Fix the plan-invalidation mechanism to treat regclass constants that refer to

a relation as a reason to invalidate a plan when the relation changes.  This
handles scenarios such as dropping/recreating a sequence that is referenced by
nextval('seq') in a cached plan.  Rather than teach plancache.c all about
digging through plan trees to find regclass Consts, we charge the planner's
setrefs.c with making a list of the relation OIDs on which each plan depends.
That way the list can be built cheaply during a plan tree traversal that has
to happen anyway.  Per bug #3662 and subsequent discussion.