Fix ALTER SUBSCRIPTION ... SET PUBLICATION ... command.

The problem is that ALTER SUBSCRIPTION ... SET PUBLICATION ... will lead
to restarting of apply worker and after the restart, the apply worker will
use the existing slot and replication origin corresponding to the
subscription. Now, it is possible that before the restart, the origin has
not been updated, and the WAL start location points to a location before
where PUBLICATION pointed to by SET PUBLICATION doesn't exist, and that
can lead to an error like: "ERROR:  publication "pub1" does not exist".
Once this error occurs, apply worker will never be able to proceed and
will always return the same error.

We decided to skip loading the publication if the publication does not
exist. The publication is loaded later and updates the relation entry when
the publication gets created.

We decided not to backpatch this as this is a behaviour change, and we don't
see field reports. This problem has been found by intermittent buildfarm
failures.

Author: vignesh C <vignesh21@gmail.com>
Reviewed-by: Dilip Kumar <dilipbalaut@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Discussion: https://postgr.es/m/flat/CALDaNm0-n8FGAorM%2BbTxkzn%2BAOUyx5%3DL_XmnvOP6T24%2B-NcBKg%40mail.gmail.com
Discussion: https://postgr.es/m/CAA4eK1+T-ETXeRM4DHWzGxBpKafLCp__5bPA_QZfFQp7-0wj4Q@mail.gmail.com

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