Duplicate cost calculation by TableSkewCostFunction when balancing by table

When using balance by table, the StochasticLoadBalancer will create the cluster state according to the regions distribution of only one table. As a result,  the TableSkewCostFunction should be replaced by the RegionCountSkewCostFunction when the table count of the cluster state is less than 2.

The most important problem is that,  TableSkewCostFunction will cause unnecessary calculation steps when there is only one table.

For example，we have 40000+ regions for one table in one group, balance this table may calculate lots of steps, but we can avoid duplicate computing costs of TableSkewCostFunction while computing cost of RegionCountSkewCostFunction and balancing by table.