JIRA: https://jira.spring.io/browse/INT-3724
Add support for `CompletableFuture<?>` return types on gateway methods, if JDK8 is being used.
- If the return type is exactly `CompletableFuture` and an async executor is provided, use `CompletableFuture.supplyAsync()`
- If there is no return async executor, return types can be `CompletableFuture` or a subclass and the flow can return such a future.
- Also fixes a problem for return type `Future<Message<?>>` with no async executor; previously this caused a `ClassCastException`
because the gateway returned the message - it assumed such return types would always run on an excutor.
We can consider back-porting this last part, but nobody has complained.
CompletableFuture Docs
Fix typos, polishing for JavaDocs and some code style polishing
5e9624f2cf