MB-11785: Disallow opening multiple conns on same socket

Change-Id: I3da9eb286587ff6d831f93afcd743fcc0c87bdd6
Reviewed-on: http://review.couchbase.org/39843
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>