use estimate stat instead of backfillRemaining for waiting

... for data move. because backfillRemaining returns only
backfill items vs. all items

Change-Id: Ic10bb6e0893fd49c9019724bfad728855ce3300d
Reviewed-on: http://review.couchbase.org/36282
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>