Recursive CTE must properly respect parenthesis

Now when parenthesis (brackets) is properly supported for query expressions numerous limitations imposed on recursive CTE must be lifted. In particular,

• mix of UNION and UNION ALL must be allowed
• INTERSECT and EXCEPT don't have to be wrapped into derived tables.
  At same time the rules of partitioning a recursive CTE specification for non-recursive and recursive part must strictly follow the SQL standard. This partitioning must be ascertained by the top level operation of the CTE specification and this operation must be either UNION or UNION ALL