Relation options can define a lock mode other than AccessExclusiveMode
since 47167b79, but modules defining custom relation options did not
really have a way to enforce that.  Correct that by extending the
current API set so as modules can define a custom lock mode.

Author: Michael Paquier
Reviewed-by: Kuntal Ghosh
Discussion: https://postgr.es/m/20190920013831.GD1844@paquier.xyz