Added async support to the interface.

[benelori]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Documented?	no
Fixed tickets	#970
License	MIT

This PR adds support for ReactPHP promises, because the wait() method of the current executor is synchronous and it will block the event loop.

This is more like a hack btw, because it's the first time I dove into the project and I needed something to work for my current work, which has ReactPHP.

I have issues with adding yet another flag to be honest and I'm not sure how to solve the issue with the controller, which will need to something like this in the createResponse method

if ($payload instanceof Promise) {
    return new ReactPromise(
        function ($resolve) use ($payload) {
            $payload
                ->then(
                    function (ExecutionResult $result) use ($resolve) {
                        $resolve(new JsonResponse($result->toArray(), 200));
                    },
                    function ($error) {
                        $reject($error);
                    }
                );
        }
    );
}

I would be glad to implement this properly, so I would like to ask for some opinions, on how this actually should look like