NCBC-568: Update IVBucket to support more than one replica

Change-Id: I739b2cf18a7bd7f963f2b5265ec3b8df873ccfbf
Reviewed-on: http://review.couchbase.org/39991
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>