JIRA: https://jira.spring.io/browse/INT-832
When we scan directory recursively for files (e.g. `WatchServiceDirectoryScanner`), it can be useful to get access to the relative path from the `Message`, e.g. on the `FileWritingMessageHandler` side to restore the original structure
* Add a `FileHeaders.FILENAME` into the outbound `Message` from the `FileReadingMessageSource`
* The result of that header is like a removal of leading `this.directory.getAbsolutePath()` in the target `File.getAbsolutePath()`.
In case of not recursion we get just only regular file name.
* Introduce `FileHeaders.RELATIVE_PATH`
* Populated `FileHeaders.RELATIVE_PATH`, `FileHeaders.FILENAME`, `FileHeaders.ORIGINAL_FILE` in the `FileReadingMessageSource`
* File `FileTailingMessageProducerSupport` to populate `FileHeaders` properly
* Introduce ctor for the `LastModifiedFileListFilter` for better Java configuration experience
* Add docs for changes
Doc Polishing
8f0fa3468f