Avoid using tuple from syscache for update of pg_database.datfrozenxid

Avoid using tuple from syscache for update of pg_database.datfrozenxid

pg_database.datfrozenxid gets updated using an in-place update at the
end of vacuum or autovacuum.  Since 96cdeae, as pg_database has a toast
relation, it is possible for a pg_database tuple to have toast values
if there is a large set of ACLs in place.  In such a case, the in-place
update would fail because of the flattening of the toast values done for
the catcache entry fetched.  Instead of using a copy from the catcache,
this changes the logic to fetch the copy of the tuple by directly
scanning pg_database.

Per the lack of complaints on the matter, no backpatch is done.  Note
that before 96cdeae, attempting to insert such a tuple to pg_database
would cause a "row is too big" error, so the end-of-vacuum problem was
not reachable.

Author: Ashwin Agrawal, Junfeng Yang
Discussion: https://postgr.es/m/DM5PR0501MB38800D9E4605BCA72DD35557CCE10@DM5PR0501MB3880.namprd05.prod.outlook.com