Add more statistics information for StoredValue objects

Track the number of them, their size and overhead

Change-Id: Ibafde84fd950c76ec9878fa903bf593836886b54
Reviewed-on: http://review.couchbase.org/37433
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>