added infrastructure for saslauthd auth

Change-Id: I6285655e0f6597f6f6f72c159d88e5754d467ad2
Reviewed-on: http://review.couchbase.org/44071
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>