A followup patch will add a SKIP_LOCKED option. To avoid introducing
evermore arguments, breaking existing callers each time, introduce a
flags argument. This'll no doubt break a few external users...

Also change the MISSING_OK behaviour so a DEBUG1 debug message is
emitted when a relation is not found.

Author: Nathan Bossart
Reviewed-By: Michael Paquier and Andres Freund
Discussion: https://postgr.es/m/20180306005349.b65whmvj7z6hbe2y@alap3.anarazel.de