* Add new correlation endpoint options
* Add `messageStore` to the `PriorityChannelSpec`
* Add generic to `EnricherSpec#property`
* Remove `type` from `EnricherSpec.headerExpression`
* Add `initialDelay` for `PeriodicTriggerSpec`
* Fix `IntegrationFlowBuilder` bugs, when there is no relevant the end of the flow
* Add embedded MongoDb support for tests
* Add `priority` test with `MongoDbChannelMessageStore`
* Add generic argument to `GenericEndpointSpec`.
Since `EndpointSpec#get()` returns `Tuple` with `EndpointFactoryBean` and `MessageHandler` objects
it is useful to get deal with specific generic for further `MessageHandler` configuration within `EndpointConfigurer` lambda
* Change DSL-methods to get deal deal with those generics.
* Add `FileWritingMessageHandler` test to demonstrate it.
* Fix bug around double `MessageHandler` bean registration from `DslIntegrationConfigurationInitializer`, when handler is a reference
to existing bean.
* Remove generic argument from `GenericEndpointSpec`: it isn't necessary
* Add check for `one-way` flow in case the last component isn't `MessageProducer`
* Provide `id`-aware DSL methods for `MessageChannelSpec`
* Rework `DslIntegrationConfigurationInitializer` to delegate to `IntegrationFlowBeanPostProcessor`
to allow to use components from `IntegrationFlowBuilder` via `@Autowired`
