WIP patch: Improve relation size functions such as pg_relation_size() to avoid producing an error when called against a no longer visible relation

Attached is a patch that addresses the todo item "Improve relation
size functions such as pg_relation_size() to avoid producing an error
when called against a no longer visible relation."

http://archives.postgresql.org/pgsql-general/2010-10/msg00332.php

Instead of returning an error, they now return NULL if the OID is
found in pg_class when using SnapshotAny. I only applied it to 4
functions: select pg_relation_size, pg_total_relation_size,
pg_table_size and pg_indexes_size. These are the ones that were using
relation_open(). I changed them to using try_relation_open(). For
three of them I had to move the try_relation_open() call up one level
in the call stack and change the parameter types for some support
functions from Oid to Relation. They now also call a new function
relation_recently_dead() which is what checks for relation in
SnapshotAny. All regression tests passed.

Is SnapshotAny the snapshot I should be using? It seems to get the
correct results. I can drop a table and I get NULL. Then after a
vacuumdb it returns an error.