code: public static ConjunctFuture<Void> waitForAll(Collection<? extends CompletableFuture<?>> futures) {
  		checkNotNull(futures, "futures");
  
  		return new WaitingConjunctFuture(futures);
  	}
docs: Creates a future that is complete once all of the given futures have completed.
  The future fails (completes exceptionally) once one of the given futures
  fails.
  
  <p>The ConjunctFuture gives access to how many Futures have already
  completed successfully, via {@link ConjunctFuture#getNumFuturesCompleted()}.
  
  @param futures The futures to wait on. No null entries are allowed.
  @return The WaitingFuture that completes once all given futures are complete (or one fails).