Calculate backfill remaining more accurately

We now have a couchstore api that will allow us to count a range
of sequence numbers to count.

Change-Id: I65e95372d7652345fa6ee4e4c0722475ac88d61e
Reviewed-on: http://review.couchbase.org/33164
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>