protected List<Pair<ServerName, Long>> getDeadServers(long since) {
    if (master.getServerManager() == null) {
      return Collections.emptyList();
    }

    return master.getServerManager().getDeadServers().copyDeadServersSince(since);
  }
Get the servers which died since a given timestamp. protected because it can be subclassed by
the tests.
hbase-server/src/main/java/org/apache/hadoop/hbase/master/ClusterStatusPublisher.java