JIRA: https://jira.spring.io/browse/INT-4430
When Iterator-based `FileSplitter` splits the file, and an exception
throws in downstream flow (in the same thread), the exception propagates
to the caller leaving underlying file reader opened.
This commit changes `AbstractMessageSplitter` the way, that,
when any exception happens, if Iterator implements `java.io.Closeable`,
its `close()` method will be called before propagating exception.
Also `FileSplitter`'s underlying iterator implements `Closeable` now.
* Make `CloseableIterator` to follow `Closeable` contract.
Now `CloseableIterator.close()` declares `IOException` and can be used
as base interface for `FunctionIterator`.
* Adjust tests.
Adjust tests to reflect the fact that we call `close()` on the reader
one more time in the end of iterator.
**Cherry-pick to 5.0.x and 4.3.x**
7f25cba262