INT-3199: Fix inconsistency for candidate methods

JIRA: https://jira.springsource.org/browse/INT-3199

* Set `TypeDescriptor` to `Void.class` as default `targetParameterType` for falling-back
* Change `fall-thru` only to one method:
* Extract generic type from `Massage` parameter, so now
``` m1(Message<String> message);
    m2(String payload);
```
are equal by `payload type` and similar POJO causes ambiguity configuration exception
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
Polishing
INT-3199: Add `handlerMessageMethods`

* Introduce separate properties:
 - `handlerMessageMethods` for methods with `Message` parameter
 - `handlerMethod`, if there is only one candidate after configuration
* Polishing several tests according new state of `MessagingMethodInvokerHelper`'s properties