pgsql: WAL log unchanged toasted replica identity key attributes.

WAL log unchanged toasted replica identity key attributes.

Currently, during UPDATE, the unchanged replica identity key attributes
are not logged separately because they are getting logged as part of the
new tuple. But if they are stored externally then the untoasted values are
not getting logged as part of the new tuple and logical replication won't
be able to replicate such UPDATEs. So we need to log such attributes as
part of the old_key_tuple during UPDATE.

Reported-by: Haiying Tang
Author: Dilip Kumar and Amit Kapila
Reviewed-by: Alvaro Herrera, Haiying Tang, Andres Freund
Backpatch-through: 10
Discussion: https://postgr.es/m/OS0PR01MB611342D0A92D4F4BF26C0F47FB229@OS0PR01MB6113.jpnprd01.prod.outlook.com

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/1cd5802ac69a1831b1a1c3b7c69dd99ae80f0b5b

Modified Files
--------------
contrib/test_decoding/expected/toast.out | 2 +-
doc/src/sgml/ref/alter_table.sgml | 9 +-
src/backend/access/heap/heapam.c | 176 ++++++++++++++++++++-----------
3 files changed, 122 insertions(+), 65 deletions(-)