Refactor code in charge of grabbing the relations of a subscription

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