Much restructing of rowsecurity test case

Much restructing of rowsecurity test case

- Some problems related to inherited tables fixed by ensuring column ordering.
- ORDER BY clauses added at some other places to ensure consistent row ordering.
- Changes related to TABLESAMPLE accepted as XL returns more rows than PG
- SAVEPOINTs removed and replaced by transaction blocks as XL does not support
  subtransaction
- NOTICEs are not displayed in XL
- Append is pushed down to the remote node now that we impose stricter
  restrictions on inheritance