Have CLUSTER ignore partitions not owned by caller

Have CLUSTER ignore partitions not owned by caller

If a partitioned table has partitions owned by roles other than the
owner of the partitioned table, don't include them in the to-be-
clustered list.  This is similar to what VACUUM FULL does (except we do
it sooner, because there is no reason to postpone it).  Add a simple
test to verify that only owned partitions are clustered.

While at it, change memory context switch-and-back to occur once per
partition instead of outside of the loop.

Author: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-by: Zhihong Yu <zyu@yugabyte.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/20220411140609.GF26620@telsasoft.com