private void remove(final ServerName sn) {
    synchronized (this.drainingServers) {
      this.drainingServers.remove(sn);
      this.serverManager.removeServerFromDrainList(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