This patch changes the load balancing simulator so that it computes
table load based on tablet sizes instead of tablet count.
best_shard_overcommit measured minimal allowed overcommit in cases
where the number of tablets can not be evenly distributed across
all the available shards. This is still the case, but instead of
computing it as an integer div_ceil() of the average shard load,
it is now computed by allocating the tablet sizes using the
largest-tablet-first method. From these, we can get the lowest
overcommit for the given set of nodes, shards and tablet sizes.
d7cfaf3f84