Make use of plancache module for SPI plans. In particular, since plpgsql

Make use of plancache module for SPI plans.  In particular, since plpgsql
uses SPI plans, this finally fixes the ancient gotcha that you can't
drop and recreate a temp table used by a plpgsql function.

Along the way, clean up SPI's API a little bit by declaring SPI plan
pointers as "SPIPlanPtr" instead of "void *".  This is cosmetic but
helps to forestall simple programming mistakes.  (I have changed some
but not all of the callers to match; there are still some "void *"'s
in contrib and the PL's.  This is intentional so that we can see if
anyone's compiler complains about it.)