Implementation of the union processor. This can be enhanced later on.
Currently, it does the following: Identify if both the subqueries of UNION
are map-only. Store that fact in the unionDesc/UnionOperator. If either of
the sub-query involves a map-reduce job, a FS is introduced on top of the
UNION. This can be later optimized to clone all the operators above the
The parse Context is not changed.