Do not lock tables in get_tables_to_repack

Hi hackers,

When doing a whole database repack, we build a list of repackable
tables and take a lock on them to prevent concurrent drops. But
concurrent drops can always happen after we build the list because
we process each table in a separate transaction. The
ConditionalLockRelationOid() also makes the default behavior like
SKIP_LOCKED, which is unexpected.

To remove the locks, we need to make repack_is_permitted_for_relation()
handles concurrent drops correctly: it should not report an error
when failing to search the syscache in pg_class_aclcheck(). Use
pg_class_aclcheck_ext() instead to detect a concurrent drop. Also
check the return value of get_rel_name().

Thoughts?

--
Regards,
ChangAo Chen