JIRA: https://jira.spring.io/browse/INT-4506
Capture the actual splitter result in the `FunctionIterator and close it if necessary.
**cherry-pick to 5.0.x**
Add Closeable to proxy in test; remove AutoCloseable check.
Closeable extends AutoCloseable.
Fix javadocs; test for AutoCloseable instead of Closeable
Polishing - PR comments; tighten up API.
A `CloseableIterator` is now an `AutoCloseable` with an overridden
`close()`` (no exception) to avoid lint problem.
* Polishing omissions, code style and `@author`
**Cherry-pick to 5.0.x**
bebde260db