INTSAMPLES-136 Async GW Promise/ListenableFuture

JIRA: https://jira.spring.io/browse/INTSAMPLES-136
Good. How about to show here `@EnableReactor` from `reactor-spring-context` since it is a sample project ?
https://github.com/spring-projects/spring-integration/blob/master/spring-integration-core/src/test/java/org/springframework/integration/configuration/EnableIntegrationTests.java#L830