added datatype/hello extensions constants

Change-Id: I6fc0d518c4875ce53c639439ab130400290820b3
Reviewed-on: http://review.couchbase.org/37553
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>