@Override
            public List<Future> call() {
                for (FutureResponse task : workingTasks) {
                    task.get();
                }
                return null;
            }
Performs an async processing. Note that the given request cannot be simultaneously
used in multiple such processings - a clone must be created for each.
container-core/src/main/java/com/yahoo/processing/execution/AsyncExecution.java