IMPALA-1005: Print consistent plan fragment ids in explain plan and runtime profile.

Change-Id: I63b59a896dc9dc0c9ed1d5e889f7b5626ba61202
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3037
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins