INT-3149: Cleanup of the What's New

JIRA: https://jira.springsource.org/browse/INT-3149

* Add a note about `GatewayMethodMetadata`
* Squash several sections to one by type
* Reorder by importance/interestingness
* Remove a note about `MessagingException` for `RetryAdvice`,
as the last one doesn't wrap exceptions to `MessagingException` on each retry