Improve data structure and algorithm for BalanceClusterState to improve computation speed for large cluster

With ~800 node and ~500 regions per node on our large production cluster, balancer cannot complete within hours even after we just add 2% servers after maintenance. 

The unit tests with larger number of regions are taking longer and longer with changes to balancer with recent changes too, evident with the increment of the time limit recent PR's included.

It is time to replace some of the data structure for better time complexity including:

int[][] regionsPerServer; // serverIndex -> region list

int[][] regionsPerHost; // hostIndex -> list of regions
int[][] regionsPerRack; // rackIndex -> region list

// serverIndex -> sorted list of regions by primary region index
ArrayList<HashSet<Integer>> primariesOfRegionsPerServer;

// hostIndex -> sorted list of regions by primary region index
int[][] primariesOfRegionsPerHost;

// rackIndex -> sorted list of regions by primary region index
int[][] primariesOfRegionsPerRack;

Areas of algorithm improvement include:

1. O(n ) to O(1) time to  lookup or update per server/host/rack for every move test iteration.(n = number of regions per server/host/rack).
2. O(n ) to O(1) time for reserse lookup of region index from primary index.
3. Recomputation of primaryRegionCountSkewCostFunction reduced from O(n ) to O(1)