use os:timestamp instead of erlang:now

Change-Id: I0d139c0c0b880ae405e7147d71615fbef80db13b
Reviewed-on: http://review.couchbase.org/40092
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>