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.

INT-3506 Fix Object returnType

INT-3506 Polishing - PR Comments

Perform dummy invocations of `submit` and `submitListenable` to
determine the actual return types so that we can determine at
runtime whether the executor will return a type that is compatible
with the method return type. If not, run on the caller's thread.

Add DEBUG Log If Incompatible Future<?>

INT-3506 Add Support for MessagingGateway

Add a constant to indicate no executor.

Add tests.

INT-3506 Polishing and Docs

- Docbook
- XSD
- Change test to send calling thread in payload so we can determine whether
   we need to return a Future or not; previously relied on the thread name
   which was brittle.

Polishing JavaDocs.
Change `amqp.xml` to use `org.springframework.amqp.support.AmqpHeaders` instead of an old one.