MB-8988: Sequence numbers are assigned by the CheckpointManager

Change-Id: If3fdf08e1091ad36ea8bcd30b10cd3803db009ae
Reviewed-on: http://review.couchbase.org/29226
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>