Fix decimal bugs.

Fix overflow handling in a few cases and add decimal as hs2 type.

Change-Id: Ifde1988365f6be961e7eb7404ed37d7bbaab875c
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2531
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins