MB-9866 StoredValue now contains queued_item instance.

Use reference-counted Item (queued_item) class inside StoredValue
class so that CheckPoint Manger and Hashtable can share keys and
metadata of items instead of keeping separate copies.
This will greatly decrease memory usage for cases where there
is big number of small sized items.

Change-Id: I304ef139dc08f99535e41abee81d4fa285a8b228
Reviewed-on: http://review.couchbase.org/32700
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>