INT-3506 Async Gateway Improvements

JIRA: https://jira.spring.io/browse/INT-3506
JIRA: https://jira.spring.io/browse/INT-3428

Support flows downstream of the gateway that support
returning a `Future<?>` payload.

Currently, any method that returns a type that is assignable
to `Future<?>` runs async and returns a `FutureTask<?>`.

This prevents a service-interface method that returns a
custom `Future<?>` object from being invoked without wrapping
that `Future<?>` in a `FutureTask<?>`.

Allow the async-executor to be set to `null` causing any method
returning `Future<?>` to run on the calling thread.

Add support for `ListenableFuture<?>`.

If the return type is a `RunnableFuture`, `ListenableFuture` or
`Future`, or exactly a `FutureTask` or `ListenableFutureTask`, run the flow
on the executor (if present); otherwise run on the calling thread.

Still needs docs.
That's true and it doesn't matter, if we disable executor. So, this might be just in the wrong place. By it is related to our general restriction concern anyway