MB-11104 Return the correct start seq number on cursor registration

When the cursor is registered, the start seq number to be returned
to the caller should be equal to the next item to be fetched
thorough a cursor.

Change-Id: I3f9aa8e40be6e5f3dafd3c218f17cb837bb4d96d
Reviewed-on: http://review.couchbase.org/37465
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>