IMPALA-1121: Fix joins on decimal columns with different precision/scale.

Change-Id: Ibac69763e28ad33ef41d000b5dd74fc73c74b73a
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3726
Reviewed-by: Marcel Kornacker <marcel@cloudera.com>
Tested-by: jenkins
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3739
Reviewed-by: Nong Li <nong@cloudera.com>