Add a query option to control query timeout.

SET QUERY_TIMEOUT_S=30 for a 30s idle query timeout. The maximum and
default query timeouts are both controlled by --idle_query_timeout, if > 0.

Change-Id: I2d2a7fd982cbd7ec34c0496dffa146d8a10adabb
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3617
Reviewed-by: Henry Robinson <henry@cloudera.com>
Tested-by: jenkins