INT-2243: Delayer: add support of 'expression'

Previously, the `<delayer>` provided only `delay-header-name` attribute.
In complex cases there was need to precede it with `<header-enricher>`.

* Add support for 'expression' attribute and sub-element
* Deprecate `delay-header-name`
* Make `DelayHandler.DelayedMessageWrapper` *public* to allow access for Messages in the Store
* Add tests
* Add 'What's new' section
* Polishing Delayer's doc regarding new abilities

JIRA: https://jira.springsource.org/browse/INT-2243, https://jira.springsource.org/browse/INT-3049