code: public CompletableFuture<Void> close() {
      CompletableFuture[] futures = new CompletableFuture[connections.size()];
  
      for (CompletableFuture<Connection> future : this.connectionFutures.values()) {
        future.cancel(false);
      }
  
      int i = 0;
      for (Connection connection : connections.values()) {
        futures[i++] = connection.close();
      }
  
      return CompletableFuture.allOf(futures);
    }
docs: Closes the connection manager.
  
  @return A completable future to be completed once the connection manager is closed.