code: public final <T> CompletionStage<T> completedStage(T value) {
          CompletableCompletionStage<T> result = createCompletionStage();
          result.complete(value);
          return result;
      }
docs: Returns a new CompletionStage that is already completed with
  the given value.
  
  @param value the value
  @param <T> the type of the value
  @return the completed CompletionStage