Decrement backfillRemaining when we send the item

Previously we decremented backfillRemaining when the item was
queued to be sent. This change decrements backfillRemaining when
the item is actually sent.

Change-Id: I3785b15e412409481ee5defcb83517b35656e831
Reviewed-on: http://review.couchbase.org/33163
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>