Make new partitions with parent's persistence during MERGE/SPLIT

Make new partitions with parent's persistence during MERGE/SPLIT

The createPartitionTable() function is responsible for creating new partitions
for ALTER TABLE ... MERGE PARTITIONS, and ALTER TABLE ... SPLIT PARTITION
commands.  It emulates the behaviour of CREATE TABLE ... (LIKE ...), where
new table persistence should be specified by the user.  In the table
partitioning persistent of the partition and its parent must match.  So, this
commit makes createPartitionTable() copy the persistence of the parent
partition.

Also, this commit makes createPartitionTable() recheck the persistence after
the new table creation.  This is needed because persistence might be affected
by pg_temp in search_path.

This commit also changes the signature of createPartitionTable() making it
take the parent's Relation itself instead of the name of the parent relation,
and return the Relation of new partition.  That doesn't lead to
complications, because both callers have the parent table open and need to
open the new partition.

Reported-by: Alexander Lakhin
Discussion: https://postgr.es/m/dbc8b96c-3cf0-d1ee-860d-0e491da20485%40gmail.com
Author: Dmitry Koval
Reviewed-by: Alexander Korotkov, Robert Haas, Justin Pryzby, Pavel Borisov