MB-11488 Fix to the memory leak in Stream instance.

Item instances in Stream instance's queue should be freed within
Stream de-constructor.

Change-Id: I026b0769d73d81242b0288c34592feb682398c4b
Reviewed-on: http://review.couchbase.org/38950
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>