Setting batch (cells limit) on scan may screw up readless increments summation

Inside IncrementSummingScanner we advance on baseScanner using passed limit. But we may grab cells up to the middle of the the group of delta-increment-puts of the counter. See todo in the code for more details - it is easier to explain there right at the code.

marked as "major" as we don't set batch or limit explicitly so far, so it probably doesn't affect the correctness at the moment. Will need to dig more.