[YSQL] Rows inserted by DDL are not visible to the encompassing repeatable read transaction

[mbautin]

Jira Link: DB-4822
This is an issue with running DDL statements inside their separate DocDB-level transactions as implemented by #3108.

E.g. from yb_pg_rowsecurity.sql:

--
-- Collation support
-- Running in serializable mode to pick up rows created within the separate DDL transaction.
--
BEGIN;
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
CREATE TABLE coll_t (c) AS VALUES ('bar'::text);
CREATE POLICY coll_p ON coll_t USING (c < ('foo'::text COLLATE "C"));
ALTER TABLE coll_t ENABLE ROW LEVEL SECURITY;
GRANT SELECT ON coll_t TO regress_rls_alice;
SELECT (string_to_array(polqual, ':'))[7] AS inputcollid FROM pg_policy WHERE polrelid = 'coll_t'::regclass;
SET SESSION AUTHORIZATION regress_rls_alice;
SELECT * FROM coll_t;
ROLLBACK;

Here, we were required to set isolation level to SERIALIZABLE, or SELECT * FROM coll_t; would see no rows, as its read point was selected before the table coll_t was created, and the rows inserted as part of the creation of that table are not visible to that read point.