Calling CCI is problematic during parallel operations involving columnar tables

Also applies to older versions, with a slightly different error message:

```sql
CREATE TABLE sampletable (x numeric);
INSERT INTO sampletable SELECT random() * 10000 FROM generate_series(1,10000000);

-- since there are some parallel workers reading from heap, and CommandCounterIncrement
-- is not expected to be called during parallel operations
CREATE TABLE sampletable_columnar2 USING columnar AS SELECT * FROM sampletable ORDER BY 1 ASC;
ERROR: cannot start commands during a parallel operation
```