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-test/src/test/java/org/springframework/integration/test/mock/MockMessageSourceTests-context.xml

@@ -3,7 +3,7 @@
 	   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 	   xmlns:int="http://www.springframework.org/schema/integration"
 	   xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
-http://www.springframework.org/schema/integration http://www.springframework.org/schema/integration/spring-integration.xsd">
+						http://www.springframework.org/schema/integration http://www.springframework.org/schema/integration/spring-integration.xsd">
 
 	<int:inbound-channel-adapter id="inboundChannelAdapter" channel="results">
 		<bean class="org.springframework.integration.test.mock.MockIntegration" factory-method="mockMessageSource">

spring-integration-test/src/test/java/org/springframework/integration/test/mock/MockMessageSourceTests.java

@@ -57,7 +57,7 @@ import org.springframework.test.context.junit4.SpringRunner;
  */
 @RunWith(SpringRunner.class)
 @ContextConfiguration(classes = MockMessageSourceTests.Config.class)
-@SpringIntegrationTest(noAutoStartup = {"inboundChannelAdapter", "*Source*"})
+@SpringIntegrationTest(noAutoStartup = { "inboundChannelAdapter", "*Source*" })
 @DirtiesContext
 public class MockMessageSourceTests {
 
@@ -76,7 +76,7 @@ public class MockMessageSourceTests {
 	@After
 	public void tearDown() {
 		this.mockIntegrationContext.resetBeans("mySourceEndpoint", "inboundChannelAdapter");
-		results.purge(null);
+		this.results.purge(null);
 	}
 
 	@Test
@@ -103,8 +103,11 @@ public class MockMessageSourceTests {
 
 	@Test
 	public void testMockMessageSourceInConfig() {
-		this.applicationContext.getBean("mockMessageSourceTests.Config.testingMessageSource.inboundChannelAdapter",
-				Lifecycle.class).start();
+		Lifecycle channelAdapter =
+				this.applicationContext
+						.getBean("mockMessageSourceTests.Config.testingMessageSource.inboundChannelAdapter",
+								Lifecycle.class);
+		channelAdapter.start();
 
 		Message<?> receive = this.results.receive(10_000);
 		assertThat(receive).isNotNull();
@@ -120,13 +123,13 @@ public class MockMessageSourceTests {
 			assertThat(receive.getPayload()).isEqualTo(3);
 		}
 
-		this.applicationContext.getBean("mockMessageSourceTests.Config.testingMessageSource.inboundChannelAdapter",
-				Lifecycle.class).stop();
+		channelAdapter.stop();
 	}
 
 	@Test
 	public void testMockMessageSourceInXml() {
-		this.applicationContext.getBean("inboundChannelAdapter", Lifecycle.class).start();
+		Lifecycle channelAdapter = this.applicationContext.getBean("inboundChannelAdapter", Lifecycle.class);
+		channelAdapter.start();
 
 		Message<?> receive = this.results.receive(10_000);
 		assertThat(receive).isNotNull();
@@ -142,7 +145,7 @@ public class MockMessageSourceTests {
 			assertThat(receive.getPayload()).isEqualTo("c");
 		}
 
-		this.applicationContext.getBean("inboundChannelAdapter", Lifecycle.class).stop();
+		channelAdapter.stop();
 	}
 
 	@Test
@@ -198,6 +201,11 @@ public class MockMessageSourceTests {
 			return Pollers.fixedDelay(10);
 		}
 
+		@Bean
+		public QueueChannel results() {
+			return new QueueChannel();
+		}
+
 		@Bean
 		public IntegrationFlow myFlow() {
 			return IntegrationFlows
@@ -208,11 +216,6 @@ public class MockMessageSourceTests {
 					.get();
 		}
 
-		@Bean
-		public QueueChannel results() {
-			return new QueueChannel();
-		}
-
 		@InboundChannelAdapter(channel = "results")
 		@Bean
 		public MessageSource<Integer> testingMessageSource() {