INTSAMPLES-136 Async GW Promise/ListenableFuture

JIRA: https://jira.spring.io/browse/INTSAMPLES-136
Can be simplified:
````java
return new Environment().addDispatcher("foo", new ThreadPoolExecutorDispatcher(100, 1)));
````
Anyway I don't see reason of that, because `GatewayProxyFactoryBean` produces `Promise` based on `defaultDispatcher`, which is `reactor.dispatchers.default = threadPoolExecutor` in your properties file.