Don't generate a byseq num for a backfill item on replica vbucket

Change-Id: I46080deebd8e6baae1f7b972b5eaa3259b91a910
Reviewed-on: http://review.couchbase.org/37232
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Reviewed-by: Sriram Ganesan <sriram@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>