Tighten ACL check in repack_is_permitted_for_relation()

Hi,

(CC-ing Álvaro Herrera for some thoughts)

repack_is_permitted_for_relation() uses pg_class_aclcheck_ext() to
silently skip a concurrently-dropped relation. That's wrong for a
caller that may already hold a lock on the relation whose ACL is
checked, where missing a relation is not fine, and it makes the
single-relation REPACK cases more brittle
(https://www.postgresql.org/message-id/akPhEffRipH4isWF@nathan) So
only detect a missing relation where that's expected, following the
fix for vacuum_is_permitted_for_relation() in commit 824d5f6. The new
missing_ok behavior is limited to get_tables_to_repack() and
get_tables_to_repack_partitioned(). All other callers of
repack_is_permitted_for_relation() hold a lock on the relation that
prevents it from being concurrently dropped, so this commit also adds
an assertion to that effect.

I posted this in the thread but starting a new discussion to get some
quick thoughts:
https://www.postgresql.org/message-id/CALj2ACX3pyuRS8%2B%2B6L20cJUMRTf_qbbVp69J1btJ3y6%3D77e5gw%40mail.gmail.com

Please find the attached patch for review.

--
Bharath Rupireddy
Amazon Web Services: https://aws.amazon.com