MB-9559: Add a sequence number to items from incoming backfills

This change adds a sequence number to items that are replicated
to replica vbuckets. In order to make this possible we are also
required to add a by sequence number to the queued item class.

Change-Id: I129dec81785ba5a9767895e2f5b5c5f4d8775bc1
Reviewed-on: http://review.couchbase.org/30435
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>