Change UnionNode to open its first child in UnionNode::Open().

This patch ensures that rows are available for clients to fetch
after we advance the query to FINISHED if the coordinator
fragment is rooted at a UnionNode.

Change-Id: I9b4ad3f70b46c7e7720bdd5ca9ad85479c2cb7fd
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3139
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins