MB-9343 mc_binary:decode_packet helper function

Change-Id: Ifdb235d4c7101af2a120d159c4c3ca5ec74bec80
Reviewed-on: http://review.couchbase.org/32532
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>