XD-2060: Add jdbc source

Actually, I disagree: `MessageSource` just produces a `Message`. And it doesn't matter for this type of components if `payload` is `List` or single entry. It is just a Message. 
So, your further logic is based on condition if there is need to split or not. That's why I tipically call `input-channel` of `splitter` with some `split` suffix/prefix, but not by based on the incoming `payload`. Because `splitter` works well even if that payload isn't `List`.

Of course, it doesn't matter for this internal config of the module.
It's just a concern of responsibity by name.

Out of topic: don't you need to worry about `parallel` process of those splitted messages for better throughput? Or does module's `output` just shift them to some internal Bus' queue channel immediately?
I mean if it makes sense to make this spliiter's  `input-channel` as `ExecutorChannel`