Fix to the wrong start byseq num from checkpoint registration

Change-Id: I5a95025750d93600088b51d87f5294dbf2e9c644
Reviewed-on: http://review.couchbase.org/37567
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>