Explore byte sizes, not KV counts, for progress control

[krestenkrab]

Right now, HanoiDB uses almost exclusively KV counts as measure of progress and for controlling "enough progress". If the bottleneck of HanoiDB is IO (as it should be) then it should make things more smooth to base progress control on byte counts.

Up until now, performance measurements have been based on basho_bench runs that have equal key and value sizes. If KV sizes vary greatly, then KV count is not a good measure for how much progress the incremental merge needs to do in order to keep up with an "even" response time.

Perhaps if we have numbers for average KV sizes, we can impose on the caller that an insert of a large KV requires a larger incremental step, than insert of a smaller KV.