Consider remaining space during block blockplacement if dfs space is highly utilized

This feature is helpful in avoiding smaller nodes (i.e. heterogeneous environment) getting constantly being full when the overall space utilization is over a certain threshold. When the utilization is low, balancer can keep up, but once the average per-node byte goes over the capacity of the smaller nodes, they get full so quickly even after perfect balance.

This jira proposes an improvement that can be optionally enabled in order to slow down the rate of space usage growth of smaller nodes if the overall storage utilization is over a configured threshold. It will not replace balancer, rather will help balancer keep up. Also, the primary replica placement will not be affected. Only the replicas typically placed in a remote rack will be subject to this check.

The appropriate threshold is cluster configuration specific. There is no generally good value to set, thus it is disabled by default. We have seen cases where the threshold of 85% - 90% would help. Figuring when totalSpaceUsed / numNodes becomes close to the capacity of a smaller node is helpful in determining the threshold.