[util/stopwatch.h] fixed integer overflow
[util/stopwatch.h] fixed integer overflow

Fixed integer overflow reported by ASAN on MacOS X.
The error reported by clang version 3.8.0 (branches/release_38 262722)
looked like the following:

  error: signed integer overflow: 1422286400 * 1000 cannot be
      represented in type 'int'

Change-Id: I47af59534869ea836613089f1f872a2b078d4347
Reviewed-on: http://gerrit.cloudera.org:8080/3960
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@apache.org>
Tested-by: Kudu Jenkins

EvilMcJerkface/kudu,andrwng/kudu,InspurUSA/kudu,andrwng/kudu,andrwng/kudu,andrwng/kudu,InspurUSA/kudu,cloudera/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,helifu/kudu,InspurUSA/kudu,cloudera/kudu,InspurUSA/kudu,cloudera/kudu,andrwng/kudu,helifu/kudu,cloudera/kudu,helifu/kudu,cloudera/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,InspurUSA/kudu,cloudera/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,andrwng/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,helifu/kudu,andrwng/kudu,EvilMcJerkface/kudu,helifu/kudu,InspurUSA/kudu,helifu/kudu,cloudera/kudu,helifu/kudu,cloudera/kudu,andrwng/kudu,helifu/kudu,helifu/kudu,InspurUSA/kudu,andrwng/kudu,cloudera/kudu