JIRA: https://jira.spring.io/browse/INT-3920
Remove package and private protection from the message handler `FactoryBean` hierarchy.
Ease the creation of subclasses, for example, it was impossible to subclass
the `SplitterFactoryBean` to support default, expression, or file splitter here:
43c8c041d7 (diff-bb5468e093538789dcde1b222fe90693R62)
* Fix JavaDocs for published methods.