* Defer Messaging annotations process
The `AbstractMethodAnnotationPostProcessor` and its implementations
have a `beanFactory.getBean()` call for the `@Bean` methods with
Messaging annotations.
This is done, actually, from the
`MessagingAnnotationPostProcessor.postProcessAfterInitialization()`
which might be still too early in some scenarios, like Spring Cloud Feign
with its child application contexts being initialized from the
`FeignClientFactoryBean`, causing a `BeanCurrentlyInCreationException`
See https://stackoverflow.com/questions/54887963/beancurrentlyincreationexception-when-using-spring-integration-with-spring-cloud
* Implement a `SmartInitializingSingleton` for the `MessagingAnnotationPostProcessor`
and gather `Runnable` wrappers for newly introduced `postProcessMethodAndRegisterEndpointIfAny()`
to be called later in the `afterSingletonsInstantiated()` when context is
still in the initialization phase.
All runtime-registered beans are going to be processed normally from the
regular `postProcessAfterInitialization()`
**Cherry-pick to 5.1.x**
* * Fix unused imports in the `MessagingAnnotationsWithBeanAnnotationTests`
* * Fix `IntegrationEndpointsInitializer` in the testing framework to handle
all the possible `AbstractEndpoint` beans registration.
See its JavaDocs for more info
* Fix `AbstractCorrelatingMessageHandlerParser` and
`AbstractConsumerEndpointParser` to use bean names for `outputChannel`
and `discardChannel` instead of bean references.
Since `MessagingAnnotationPostProcessor` now registers endpoints and
beans for channels much later, than parsers, we can't rely on bean
references any more there.
* Fixes for failing tests which expected `outputChannel/discardChannel`
bean references, when it is already just their names for late binding.
* Apply some code style polishing for the affected classes.
* Add `@Nullable` for `MessageSelector` parameter in the `QueueChannel.purge()`
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