code: synchronized long getNextIndex(String server) {
          checkState(role == Role.LEADER, "role:%s", role);
          checkState(self.equals(leader), "self:%s leader:%s", self, leader);
          checkState(serverData.containsKey(server), "server:%s", server);
  
          return serverData.get(server).nextIndex;
      }
docs: Get the nextIndex for the specified {@code server} in the Raft cluster.
  
  @param server unique id of the server for which nextIndex should be returned
  @return log index >= 0 of the nextIndex for the server