Don't assign checkpoint start/end/dummy items a by sequence number

Assigning checkpoint meta items sequence numbers can make consumers
think that more items have been added to a vbucket when no new items
have actually been added. This change also makes it easier to predict
the high sequence number when inserting a given amount of items into
a vbucket.

Change-Id: Ib374ccda99179b93c80bdd8d36cdfb4bf0caaf4e
Reviewed-on: http://review.couchbase.org/31502
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>