Improve behavior of concurrent ALTER <relation> .. SET SCHEMA.

If the referrent of a name changes while we're waiting for the lock,
we must recheck permissons.  We also now check the relkind before
locking, since it's easy to do that long the way.

Patch by me; review by Noah Misch.

src/backend/commands/alter.c

@@ -191,7 +191,6 @@ ExecAlterObjectSchemaStmt(AlterObjectSchemaStmt *stmt)
 		case OBJECT_TABLE:
 		case OBJECT_VIEW:
 		case OBJECT_FOREIGN_TABLE:
-			CheckRelationOwnership(stmt->relation, true);
 			AlterTableNamespace(stmt->relation, stmt->newschema,
 								stmt->objectType, AccessExclusiveLock);
 			break;