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

contrib/test_decoding/expected/toast.out

@@ -77,7 +77,7 @@ SELECT substr(data, 1, 200) FROM pg_logical_slot_get_changes('regression_slot',
  table public.toasted_key: INSERT: id[integer]:1 toasted_key[text]:'1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123
  COMMIT
  BEGIN
- table public.toasted_key: UPDATE: id[integer]:1 toasted_key[text]:unchanged-toast-datum toasted_col1[text]:unchanged-toast-datum toasted_col2[text]:'987654321098765432109876543210987654321098765432109
+ table public.toasted_key: UPDATE: old-key: toasted_key[text]:'123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678
  COMMIT
  BEGIN
  table public.toasted_key: UPDATE: old-key: toasted_key[text]:'123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678

doc/src/sgml/ref/alter_table.sgml

@@ -850,10 +850,11 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
     <listitem>
      <para>
       This form changes the information which is written to the write-ahead log
-      to identify rows which are updated or deleted.  This option has no effect
-      except when logical replication is in use.
-      In all cases, no old values are logged unless at least one of the columns
-      that would be logged differs between the old and new versions of the row.
+      to identify rows which are updated or deleted.
+      In most cases, the old value of each column is only logged if it differs
+      from the new value; however, if the old value is stored externally, it is
+      always logged regardless of whether it changed.
+      This option has no effect except when logical replication is in use.
      <variablelist>
       <varlistentry>
        <term><literal>DEFAULT</literal></term>