Minor Description
There are some problems which need to fix.
1. LocalityBasedCandidateGenerator.getLowestLocalityRegionOnServer should skip empty region.
2. When use LocalityBasedCandidateGenerator to generate Cluster.Action, it should add random operation instead of pickLowestLocalityServer(cluster). Because the search function may stuck here if it always generate the same Cluster.Action.
3. getLeastLoadedTopServerForRegion should get least loaded server which have better locality than current server.