The new partitioned table capability added a new relkind, namely
RELKIND_PARTITIONED_TABLE. Update sepgsql to treat this new relkind
exactly the same way it does RELKIND_RELATION.

In addition, add regression test coverage for partitioned tables.

Issue raised by Stephen Frost and initial patch by Mike Palmiotto.
Review by Tom Lane and Robert Haas, and editorializing by me.

Discussion: https://postgr.es/m/flat/623bcaae-112e-ced0-8c22-a84f75ae0c53%40joeconway.com