pgsql: Refactor code in charge of grabbing the relations of a subscript

Refactor code in charge of grabbing the relations of a subscription

GetSubscriptionRelations() and GetSubscriptionNotReadyRelations() share
mostly the same code, which scans pg_subscription_rel and fetches all
the relations of a given subscription.  The only difference is that the
second routine looks for all the relations not in a ready state.  This
commit refactors the code to use a single routine, shaving a bit of
code.

Author: Vignesh C
Reviewed-By: Kyotaro Horiguchi, Amit Kapila, Michael Paquier, Peter
Smith
Discussion: https://postgr.es/m/CALDaNm0eW-9g4G_EzHebnFT5zZoasWCS_EzZQ5BgnLZny9S=pg@mail.gmail.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/ce3049b0215b63744d11c0ce3ac6afdb67fc2ff0

Modified Files
--------------
src/backend/catalog/pg_subscription.c       | 70 +++++------------------------
src/backend/commands/subscriptioncmds.c     |  4 +-
src/backend/replication/logical/tablesync.c |  2 +-
src/include/catalog/pg_subscription_rel.h   |  3 +-
4 files changed, 14 insertions(+), 65 deletions(-)