pgsql: Avoid using tuple from syscache for update of pg_database.datfro

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.

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.

This issue has been originally fixed in 947789f on v14~, and there have
been reports about this problem on v12 and v13, causing failures at the
end of VACUUM. This completes the fix on all the stable branches where
pg_database can use a toast table, down to 12.

Author: Ashwin Agrawal, Junfeng Yang
Discussion: https://postgr.es/m/DM5PR0501MB38800D9E4605BCA72DD35557CCE10@DM5PR0501MB3880.namprd05.prod.outlook.com
Discussion: https://postgr.es/m/Y70XNVbUWQsR2Car@paquier.xyz
Backpatch-through: 12

Branch
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/c0ee6943ca1ded4b1a09be5832ef6e6bc257fb13

Modified Files
--------------
src/backend/access/heap/heapam.c | 4 ++++
src/backend/commands/vacuum.c | 21 +++++++++++++++++++--
2 files changed, 23 insertions(+), 2 deletions(-)