@Override
      public void serverAdded(ServerName sn) {
        if (drainingServers.contains(sn)) {
          serverManager.addServerToDrainList(sn);
        }
      }
Starts the tracking of draining RegionServers.
<p>
All Draining RSs will be tracked after this method is called.
hbase-server/src/main/java/org/apache/hadoop/hbase/master/DrainingServerTracker.java