INT-3312: Add support for `outputChannelName`

JIRA: https://jira.springsource.org/browse/INT-3312

* Add `setOutputChannelName` to `AbstractReplyProducingMessageHandler` and `SourcePollingChannelAdapterFactoryBean`
to allow to have 'late binding' and resolve the real channel from `AC` on component initialization, not create phase
* Add `setRequestChannelName` and `setReplyChannelName` to `ContentEnricher` for the same purpose as before
* Make `HeaderValueMessageProcessor` implementation as `public`. It is useful from JavaConfig and DSL
* Polishing parsers to use classes, not their strings