INT-3465:Content Enricher Improvements

JIRA: https://jira.spring.io/browse/INT-3465

Add support for adding/removing individual recipients to the RecipientListRouter

Modify documentation in what's new and spring-integration-4.1.xsd

Polishing

`AbstractRemoteFileOutboundGateway`: close `outputStream` before `file.delete()` to release exclusive file-lock