Don't schedule backfill if cursor isn't registered at first chk item

If the cursor is not registered at the first item in the checkpoint
then we do not need to schedule backfill. Since items get
de-duplicated in the checkpoint queues the current logic might
choose to schedule a backfill unnecessarily. This change stops the
scheduling of unnecessary backfills.

Change-Id: I11188d302be57035cfac5f1c105cbc6d7179bde6
Reviewed-on: http://review.couchbase.org/40145
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>