pgsql: Fix Alter Subscription's Add/Drop Publication behavior.

Fix Alter Subscription's Add/Drop Publication behavior.

The current refresh behavior tries to just refresh added/dropped
publications but that leads to removing wrong tables from subscription. We
can't refresh just the dropped publication because it is quite possible
that some of the tables are removed from publication by that time and now
those will remain as part of the subscription. Also, there is a chance
that the tables that were part of the publication being dropped are also
part of another publication, so we can't remove those.

So, we decided that by default, add/drop commands will also act like
REFRESH PUBLICATION which means they will refresh all the publications. We
can keep the old behavior for "add publication" but it is better to be
consistent with "drop publication".

Author: Hou Zhijie
Reviewed-by: Masahiko Sawada, Amit Kapila
Backpatch-through: 14, where it was introduced
Discussion: https://postgr.es/m/OS0PR01MB5716935D4C2CC85A6143073F94EF9@OS0PR01MB5716.jpnprd01.prod.outlook.com

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/5cfcd46e9d297eac627bf3183272713112ac5f60

Modified Files
--------------
doc/src/sgml/ref/alter_subscription.sgml | 7 +-
src/backend/commands/subscriptioncmds.c | 7 +-
src/bin/psql/tab-complete.c | 8 +--
src/test/regress/expected/subscription.out | 3 -
src/test/regress/sql/subscription.sql | 3 -
src/test/subscription/t/021_alter_sub_pub.pl | 98 ++++++++++++++++++++++++++++
6 files changed, 105 insertions(+), 21 deletions(-)