Refactor routines for subscription and publication lookups

Refactor routines for subscription and publication lookups

Those routines gain a missing_ok argument, allowing a caller to get a
NULL result instead of an error if set to true.  This is part of a
larger refactoring effort for objectaddress.c where trying to check for
non-existing objects does not result in cache lookup failures.

Author: Michael Paquier
Reviewed-by: Aleksander Alekseev, Álvaro Herrera
Discussion: https://postgr.es/m/CAB7nPqSZxrSmdHK-rny7z8mi=EAFXJ5J-0RbzDw6aus=wB5azQ@mail.gmail.com