PFA, a patch against git head. We take the AccessShareLock lock on the schema in DefineRelation now. Note that we do not want to interlock with other concurrent creations in the schema. We only want to interlock with deletion activity. So even performance wise this should not be much of an overhead in case of concurrent DDL operations to the same schema.
Adding this in DefineRelation handles creation of tables/views/types/sequences. I do not think we need to do stuff in ALTER commands, because the objects pre-exist and this issue appears to be with new objects only.
