docs: public void addChainDownloadStartedEventListener(Executor executor, ChainDownloadStartedEventListener listener) {
          peersChainDownloadStartedEventListeners.add(new ListenerRegistration<>(checkNotNull(listener), executor));
          for (Peer peer : getConnectedPeers())
              peer.addChainDownloadStartedEventListener(executor, listener);
          for (Peer peer : getPendingPeers())
              peer.addChainDownloadStartedEventListener(executor, listener);
      }