XD-2060: Add jdbc source

`fixedDalay` exactly says, that there will be only one process at a time. It isn't `fixedRate`. 

So, if your JDBC result is e.g. 1000 rows and downstream flow is single-threaded (I need to know how `MessageBus` works, sorry), the next poll may happen after non-desired time, bacause the next poll happens exactly after that `delay` and after **finish** of the previous one. 

That's why `ExecutorChannel` fixes the problem. Or it can be `QueueChannel`, if `MessageBus` can gets deal with them, of course.

Re. `order`. If you really worry about order of that data, so yes, `QueueChannel` is your case. 
From other side It isn't clear to me why you should worry about the order, if it's just a data and you produce several messages using `splitter`. Meybe there is no reason to split it, if the order is so important?

Of course, we can overcome it with the next `resequencer` module anyway...