Keep track of number of item objects

Change-Id: I7a8a5d037f3f3b5243d1257cb400a8030d125a9d
Reviewed-on: http://review.couchbase.org/37583
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>