Make StochasticLoadBalancer more like Simulated annealing

* Added a base class for computing a cost.
* Added cost for reion count skew
* Added cost for moving
* Added cost for locality
* added base class for costs based upon region load moving average
* added cost for read request
* added cost for write request
* added cost for storefile size
* added cost for memstore size.
* moved away from DescriptiveStats
* Added a base class for picking servers and regions
* Added region picker based upon region load
* Added region picker based upon locality
* Added random region picker