Allow MergeNode with constant selects to run correctly on multiple fragment instances.

Change-Id: I0b1ff27f591366b960aa944fadabbb4b35f4b9b4
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2832
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins
Fix 'summary' when exch map is not set

Change-Id: I66d9987f45f6cee045a300f86de357a2761929d7
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3000
Reviewed-by: Henry Robinson <henry@cloudera.com>
Tested-by: jenkins