private FutureResponse getFutureResponse(Callable<Response> callable, Request request) {
        FutureResponse future = new FutureResponse(callable, execution, request);
        try {
            executorMain.execute(future.delegate());
        } catch (RejectedExecutionException e) {
            future.delegate().run();
        }
        return future;
    }
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