INT-3199: Polishing and further fixing

* Address PR's comments
* Provide more friendly exception message for `Void.class` method
* Remove the logic around `@ServiceActivator`:
 - one part was fixed within INT-3114
 - another for `RequestReplyExchanger#exchange` doesn't make sense as it is good candidate
* Add fallback to `Iterator.class` method for `Iterable` payloads
* Add tests