@Override
        public Object run() {
          abortRegionServer(reason, cause);
          return null;
        }
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-server/src/test/java/org/apache/hadoop/hbase/SingleProcessHBaseCluster.java