Make region balancing parallel with balancer.balanceCluster()

balancer.balanceCluster() generates RegionPlans for HMaster.balance() to execute.
We don't retract any RegionPlan in balancer.balanceCluster().
In the near future, more complex algorithm would be introduced to try achieving maximum block location affinity for the regions to be moved. This means balancer.balanceCluster() would take longer to return.

This JIRA makes region balancing parallel with balancer.balanceCluster()
Meaning region balancing would be performed when balancer.balanceCluster() is still running.