protected static class BaseLoadBalancer.Cluster
An efficient array based implementation similar to ClusterState for keeping
the status of the cluster in terms of region assignment and distribution.
LoadBalancers, such as StochasticLoadBalancer uses this Cluster object because of
hundreds of thousands of hashmap manipulations are very costly, which is why this
class uses mostly indexes and arrays.
Cluster tracks a list of unassigned regions, region assignments, and the server
topology in terms of server names, hostnames and racks.