hbase load balancer needs locality awareness

Previously, HBASE-4114 implements the metrics for HFile HDFS block locality, which provides the HFile level locality information.
But in order to work with load balancer and region assignment, we need the region level locality information.

Let's define the region locality information first, which is almost the same as HFile locality index.

HRegion locality index (HRegion A, RegionServer B) =
(Total number of HDFS blocks that can be retrieved locally by the RegionServer B for the HRegion A) / ( Total number of the HDFS blocks for the Region A)
So the HRegion locality index tells us that how much locality we can get if the HMaster assign the HRegion A to the RegionServer B.

So there will be 2 steps involved to assign regions based on the locality.
1) During the cluster start up time, the master will scan the hdfs to calculate the "HRegion locality index" for each pair of HRegion and Region Server. It is pretty expensive to scan the dfs. So we only needs to do this once during the start up time.

2) During the cluster run time, each region server will update the "HRegion locality index" as metrics periodically as HBASE-4114 did. The Region Server can expose them to the Master through ZK, meta table, or just RPC messages.

Based on the "HRegion locality index", the assignment manager in the master would have a global knowledge about the region locality distribution and can run the MIN COST MAXIMUM FLOW solver to reach the global optimization.

Let's construct the graph first:
[Graph]
Imaging there is a bipartite graph and the left side is the set of regions and the right side is the set of region servers.
There is a source node which links itself to each node in the region set.
There is a sink node which is linked from each node in the region server set.

[Capacity]
The capacity between the source node and region nodes is 1.
And the capacity between the region nodes and region server nodes is also 1.
(The purpose is each region can ONLY be assigned to one region server at one time)

The capacity between the region server nodes and sink node are the avg number of regions which should be assigned each region server.
(The purpose is balance the load for each region server)

[Cost]
The cost between each region and region server is the opposite of locality index, which means the higher locality is, if region A is assigned to region server B, the lower cost it is.
The cost function could be more sophisticated when we put more metrics into account.

So after running the min-cost max flow solver, the master could assign the regions based on the global locality optimization.

Also the master should share this global view to secondary master in case the master fail over happens.
In addition, the HBASE-4491 (Locality Checker) is the tool, which is based on the same metrics, to proactively to scan dfs to calculate the global locality information in the cluster. It will help us to verify data locality information during the run time.