eliminate duplicated stats queries in initial estimation code

.. of rebalance observer

Change-Id: I5b3bb83736129e8fb218587e18bc860b974200a7
Reviewed-on: http://review.couchbase.org/38780
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>