INTSAMPLES-136 Async GW Promise/ListenableFuture

JIRA: https://jira.spring.io/browse/INTSAMPLES-136

Polishing - PR Comments

* Add `compile "org.projectreactor.spring:reactor-spring-context:$reactorSpringVersion"` dependency
to show a usage of `@EnableReactor`
* Make `async-gateway` project Java 8 compatible and replace inline implementations for callbacks to Lambdas