CBSE-1090: handle nack request in TAP_OPAQUE command

Server can possibly send out TAP_OPAQUE_ENABLE_AUTO_NACK request
with TAP_OPAQUE request. It requires clients to reponse it instead
of simply ignoring it.

Change-Id: I6a048bf94fa8e204581339e6d98a8bbe8c56e04a
Reviewed-on: http://review.couchbase.org/35912
Reviewed-by: Steve Yen <steve.yen@gmail.com>
Tested-by: Bin Cui <bin.cui@gmail.com>