added multi-bucket-demo program

Change-Id: I91a70105069824bf3575b726bfaa4ade21f7774a
Reviewed-on: http://review.couchbase.org/44549
Reviewed-by: Manik Taneja <manik@couchbase.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>