Refactor the way items are retrieved from the Checkpoint Manager

Change-Id: I98d34ee9720142c7c5d8524836bbe68284330b33
Reviewed-on: http://review.couchbase.org/29570
Tested-by: buildbot <build@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>