MB-10033 use stats to obtain file and data sizes

to determine if the compaction is needed

Change-Id: Ic11155ba494801f4751384d1f8a1d5158420945e
Reviewed-on: http://review.couchbase.org/33098
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>