* Remove generic argument from `GenericEndpointSpec`: it isn't necessary
* Add check for `one-way` flow in case the last component isn't `MessageProducer`
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.
* Provide `id`-aware DSL methods for `MessageChannelSpec`
* Rework `DslIntegrationConfigurationInitializer` to delegate to `IntegrationFlowBeanPostProcessor`
to allow to use components from `IntegrationFlowBuilder` via `@Autowired`
