Revise mechanism for getting rid of temp tables at backend shutdown.

Revise mechanism for getting rid of temp tables at backend shutdown.
Instead of grovelling through pg_class to find them, make use of the
handy dandy dependency mechanism: just delete everything that depends
on our temp schema.  Unlike the pg_class scan, the dependency mechanism
is smart enough to delete things in an order that doesn't fall foul of
any dependency restrictions.  Fixes problem reported by David Heggie:
a temp table with a serial column may cause a backend FATAL exit at
shutdown time, if it chances to try to delete the temp sequence first.