MB-11279: Fix incorrectly set cursor offset.

The offset should be the amount of items we skipped in the
checkpoint queues. Incorrectly setting the offset can lead to
stats inconsistencies when getting estimates on the number of
checkpoint items.

Change-Id: Ic5acf24425c310bd039625f73420cbf5922d8564
Reviewed-on: http://review.couchbase.org/37808
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>