Revert `IntegrationFlow` processing to `BFPP`

Add test case for `@MessageEndpoint` to et deal with channel from `IntegrationFlow`
As far as `AbstractMethodAnnotationPostProcessor` tries to resolve `channel` on endpoint creation,
the `IntegrationFlow` processing can't be done in the `BPP`, because `MessagingAnnotationPostProcessor` may parse
annotations for bean before `IntegrationFlowBeanPostProcessor` will do its stuff.