Repair bug that allowed RevalidateCachedPlan to attempt to rebuild a cached

plan before the effects of DDL executed in an immediately prior SPI operation
had been absorbed.  Per report from Chris Wood.

This patch has an unpleasant side effect of causing the number of
CommandCounterIncrement()s done by a typical plpgsql function to
approximately double.  Amelioration of the consequences of that
will be undertaken in a separate patch.