Defer Messaging annotations process (#2769)

* 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()`

spring-integration-ftp/src/test/java/org/springframework/integration/ftp/inbound/FtpStreamingMessageSourceTests.java

@@ -115,11 +115,11 @@ public class FtpStreamingMessageSourceTests extends FtpTestSupport {
 		this.data.purge(null);
 		this.metadataMap.clear();
 		this.adapter.start();
+		assertThat(this.data.receive(10000)).isNotNull();
 		received = (Message<byte[]>) this.data.receive(10000);
 		assertThat(received).isNotNull();
-		this.adapter.stop();
-
 		assertThat(received.getHeaders().get(FileHeaders.REMOTE_FILE_INFO)).isInstanceOf(FtpFileInfo.class);
+		this.adapter.stop();
 	}
 
 	@Test