MB-7583: Move checkpoint cursors when checkpoints are merged

Change-Id: Id121b2567dac41c6c29e0aaf8793f2247b33f869
Reviewed-on: http://review.couchbase.org/24150
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>