IMPALA-817: Check equality of function name in Function.equals().

Change-Id: Ib9b4ee3a21f90fdb0d7ebccd89462dc67040bd1e
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1594
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins