Create LoadBalancer perf-tests (test balancer algorithm decoupled from workload)

(Below is a quote from mbertozzi taken from an internal issue that I'm moving out here)

Add perf tools and keep monitored balancer performance (a BalancerPE-type thing).

Most of the balancers should be instantiable without requiring a mini-cluster, and it easy to create tons of RegionInfo and ServerNames with a for loop.

The balancer is just creating a map RegionInfo:ServerName.

There are two methods to test roundRobinAssignment() and retainAssignment()

Map<ServerName, List<HRegionInfo>> roundRobinAssignment(
    List<HRegionInfo> regions,
    List<ServerName> servers
  ) throws HBaseIOException; 

Map<ServerName, List<HRegionInfo>> retainAssignment(
    Map<HRegionInfo, ServerName> regions,
    List<ServerName> servers
  ) throws HBaseIOException;

There are a bunch of obvious optimization that everyone can see just by looking at the code. (like replacing array with set when we do contains/remove operations). It will be nice to have a baseline and start improving from there.