Add randomness to mocked timestamps.

Change-Id: I027802834b901aeb8bcd5f6d09ade25f0bfd9eca
Reviewed-on: http://review.couchbase.org/35283
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>