code: public void add(Endpoint follower, long lastLogIndex) {
          assert !followerStates.containsKey(follower) : "Already known follower " + follower;
          followerStates.put(follower, new FollowerState(0L, lastLogIndex + 1));
      }
docs: Add a new follower with the leader's {@code lastLogIndex}.
  Follower's {@code nextIndex} will be set to {@code lastLogIndex + 1}
  and {@code matchIndex} to 0.