IMPALA-838: Fix premature timeout of sessions

A crucial comparison was between time values with different units.

Tests didn't catch this because they only confirmed that sessions were
timed out within the correct time, not that they were *not* timed out
early.

Change-Id: Ia8c57d3d70e4702996d0225b167142b7bf88d236
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1677
Reviewed-by: Alan Choi <alan@cloudera.com>
Reviewed-by: Henry Robinson <henry@cloudera.com>
Tested-by: jenkins