Back-patch the 9.4-era commit 2636ecf78 into 9.3, as that fixes a case
where we open a relation while not holding any lock on it. It's
probably mostly safe anyway, since no other session could touch the
newly-created table; but I think CheckTableNotInUse could be fooled
if one tried.
Per testing with a patch that complains if we open a relation without
holding any lock on it. I don't plan to back-patch that patch, but we
should close the holes it identifies in all supported branches.
Discussion: https://postgr.es/m/2038.1538335244@sss.pgh.pa.us
de0bea8d4d