Avoid syncing data twice for the 'publish_via_partition_root' option.

When there are multiple publications for a subscription and one of those
publishes via the parent table by using publish_via_partition_root and the
other one directly publishes the child table, we end up copying the same
data twice during initial synchronization. The reason for this was that we
get both the parent and child tables from the publisher and try to copy
the data for both of them.

This patch extends the function pg_get_publication_tables() to take a
publication list as its input parameter. This allows us to exclude a
partition table whose ancestor is published by the same publication list.

This problem does exist in back-branches but we decide to fix it there in
a separate commit if required. The fix for back-branches requires quite
complicated changes to fetch the required table information from the
publisher as we can't update the function pg_get_publication_tables() in
back-branches. We are not sure whether we want to deviate and complicate
the code in back-branches for this problem as there are no field reports
yet.

Author: Wang wei
Reviewed-by: Peter Smith, Jacob Champion, Kuroda Hayato, Vignesh C, Osumi Takamichi, Amit Kapila
Discussion: https://postgr.es/m/OS0PR01MB57167F45D481F78CDC5986F794B99@OS0PR01MB5716.jpnprd01.prod.outlook.com

doc/src/sgml/ref/create_publication.sgml

@@ -201,6 +201,16 @@ CREATE PUBLICATION <replaceable class="parameter">name</replaceable>
           consisting of a different set of partitions.
          </para>
 
+         <para>
+          There can be a case where a subscription combines multiple
+          publications. If a partitioned table is published by any
+          subscribed publications which set
+          <literal>publish_via_partition_root</literal> = true, changes on this
+          partitioned table (or on its partitions) will be published using
+          the identity and schema of this partitioned table rather than
+          that of the individual partitions.
+         </para>
+
          <para>
           This parameter also affects how row filters and column lists are
           chosen for partitions; see below for details.