If the table being attached contained values that contradict the default
partition's partition constraint, it would fail to complain, because
CommandCounterIncrement changes in 4dba331c coupled with some bogus
coding in the existing ValidatePartitionConstraints prevented the
partition constraint from being validated after all -- or rather, it
caused to constraint to become an empty one, always succeeding.

Fix by not re-reading the OID of the default partition in
ATExecAttachPartition.  To forestall similar problems, revise the
existing code:
* rename routine from ValidatePartitionConstraints() to
  QueuePartitionConstraintValidation, to better represent what it
  actually does.
* add an Assert() to make sure that when queueing a constraint for a
  partition we're not overwriting a constraint previously queued.
* add an Assert() that we don't try to invoke the special-purpose
  validation of the default partition when attaching the default
  partition itself.

While at it, change some loops to obtain partition OIDs from
partdesc->oids rather than find_all_inheritors; reduce the lock level
of partitions being scanned from AccessExclusiveLock to ShareLock;
rewrite QueuePartitionConstraintValidation in a recursive fashion rather
than repetitive.

Author: Álvaro Herrera.  Tests written by Amit Langote
Reported-by: Rushabh Lathia
Diagnosed-by: Kyotaro HORIGUCHI, who also provided the initial fix.
Reviewed-by: Kyotaro HORIGUCHI, Amit Langote, Jeevan Ladhe
Discussion: https://postgr.es/m/CAGPqQf0W+v-Ci_qNV_5R3A=Z9LsK4+jO7LzgddRncpp_rrnJqQ@mail.gmail.com