create_immv could create an inconsistent IMMV with concurrent insert into a base table

[yugo-n]

If a base table is modified before calling create_immv in a concurrent transaction, create_immv will create an IMMV inconsistent with the latest contents of the table.

1. Modify a table in a transaction A.

   postgres=# begin ;
   BEGIN
   postgres=*# insert into yyy values (10);
   INSERT 0 1

2. Create an IMMV in a transaction B.

   postgres=# begin ;
   BEGIN
   postgres=*# select create_immv('mvyyy','select * from yyy'); -- blocked

3. Commit the transaction A.

   
   postgres=*# commit;
   COMMIT
   postgres=# select * from yyy;
   i  
   ----
   1
   2
   10
   (3 rows)

4. Commit the transaction B, the IMMV doesn't contain the inserted row.
```sql
(cont.)
NOTICE:  could not create an index on immv "mvyyy" automatically
DETAIL:  This target list does not have all the primary key columns, or this view does not contain GROUP BY or DISTINCT clause.
HINT:  Create an index on the immv for efficient incremental maintenance.
 create_immv 
-------------
           2
(1 row)

postgres=*# end;
COMMIT
postgres=# select * from mvyyy;
 i 
---
 1
 2
(2 rows)

[yugo-n]

When create_immv was called before the table modification, the resultant was correct.