MB-11104 Set a byseq num correctly in merging checkpoints.

When checkpoints are merged, the byseq values for checkpoint
meta items should be updated in addition to updating the index.

Change-Id: I0e730d62eb550f647271d467e9a7b8e749368378
Reviewed-on: http://review.couchbase.org/37353
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>