@Override
    protected void kill() {
      killedServers.add(getServerName());
      super.kill();
    }
List of RegionServers killed so far. ServerName also comprises startCode of a server, so any
restarted instances of the same server will have different ServerName and will not coincide
with past dead ones. So there's no need to cleanup this list.
hbase-testing-util/src/main/java/org/apache/hadoop/hbase/MiniHBaseCluster.java