MB-11629: Set engineSpecific for cookie to NULL (HandleDisconnect)

Change-Id: I3827143aff23c58b2bf0a548b382850dedfafd82
Reviewed-on: http://review.couchbase.org/39768
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>