INT-3114 Call MessageHandlers Directly

__For review/discusion only at this time__

It's a pretty small/straightforward change, but ...


All tests pass and I added test cases to verify we're invoked directly by the dispatcher.

One side effect is that the message history is shortened; see 

    -    assertEquals("handlerTestInputChannel,handlerTestService,testMessageHandler", reply.getHeaders().get("history").toString());
    +    assertEquals("replyingHandlerTestInputChannel,replyingHandlerTestService", reply.getHeaders().get("history").toString());

in `ServiceActivatorDefaultFrameworkMethodTests`

Before, we had channel, service-activator, ref'd bean (actual service).

now it's just channel, service-activator.

Maybe that's a good thing.