private HRegionServer getServer() {
    return (HRegionServer) server;
  }
Handles closing of a region on a region server.
<p/>
Just done the same thing with the old {@link CloseRegionHandler}, with some modifications on
fencing and retrying. But we need to keep the {@link CloseRegionHandler} as is to keep compatible
with the zk less assignment for 1.x, otherwise it is not possible to do rolling upgrade.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/handler/UnassignRegionHandler.java