Add opaque field to the close stream api

Change-Id: I1332a2badb5aa12b51645e2a6e6cbd556b4c2ca5
Reviewed-on: http://review.couchbase.org/32349
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>