INT-651: Add `Iterator` support for `Splitter`

JIRA: https://jira.spring.io/browse/INT-651
I don't think so. We already have a transitive dependency for Reactor with Spring Messaging.
And I hope we'll produce soon some Reactor support: for example `Promise` Gateway: https://github.com/artembilan/spring-integration/commit/6b503e98280d9d7dff8edc81fe16de7a927e6c66