Support OID system column in postgres_fdw.
You can use ALTER FOREIGN TABLE SET WITH OIDS on a foreign table, but the
oid column read out as zeros, because the postgres_fdw didn't know about
it. Teach postgres_fdw how to fetch it.
Etsuro Fujita, with an additional test case by me.
Discussion: <
56E90A76.
5000503@lab.ntt.co.jp>