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
INTSAMPLES-135: DSL Cafe Demo Sample

JIRA: https://jira.spring.io/browse/INTSAMPLES-135

Conflicts:
	gradle.properties

Upgrade version: Boot 1.1.5, SI 4.0.4, DSL 1.0.0.M3, SF 4.0.7