JIRA: https://jira.spring.io/browse/INT-3583
Previously, the `ref` or inner bean for an aggregator was wrapped
in a `MethodInvokingMessageGroupProcessor` for POJO aggregation logic.
Now, if the ref'd bean is a `MessageGroupProcessor`, it is used as the
output processor directly.
Also, the `SimpleMessageGroupProcessor` is added which simply returns the
collection of messages.
INT-3583: Polishing; PR Comments
Fix AsciiDoc
Revert `CorrelationMessageBarrier` deprecation
Fix Docs for wrong chapter link
bf8e79cb1e