NCBC-308: Key Exists does not release socket pool if not found

Change-Id: Id9f19887205b87aa6027ef20bb682c756fd807be
Reviewed-on: http://review.couchbase.org/29347
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Matt Ingenthron <matt@couchbase.com>
Reviewed-by: Jeffry Morris <jeffrymorris@gmail.com>
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>