JIRA: https://jira.spring.io/browse/INT-4269
* Check if `outputChannel` is `ReactiveStreamsSubscribableChannel`
to let back-pressure splitting
* Build `Flux` or `Iterator` depending in the `reactive` state
* Allow to get a size of the `iterator` if it is possible,
for example `XPathMessageSplitter`
Add tests
Fix raw type and unused import
* Add JavaDocs to the `AbstractMessageSplitter#obtainSizeIfPossible()`
* Add asserts for the `sequenceSize` populataiton in the `XPathMessageSplitter`
* Document `Stream` & `Flux` support in the splitter
Minor doc polishing
f112ecbb7e