Detect unsent DISCONNECT messages

This change uses a ChannelInterceptor (inserted at index 0) to detect
when a DISCONNECT message is precluded from being sent on the
clientInboundChannel. This can happen if another interceptor allows
a runtime exception out from preSend or returns false.

It is crucial for such messages to be processed, so when detected
they're processed still.

Issue: SPR-12218

spring-messaging/src/test/java/org/springframework/messaging/simp/config/MessageBrokerConfigurationTests.java

@@ -130,7 +130,7 @@ public class MessageBrokerConfigurationTests {
 		AbstractSubscribableChannel channel = this.customContext.getBean(
 				"clientInboundChannel", AbstractSubscribableChannel.class);
 
-		assertEquals(1, channel.getInterceptors().size());
+		assertEquals(2, channel.getInterceptors().size());
 
 		ThreadPoolTaskExecutor taskExecutor = this.customContext.getBean(
 				"clientInboundChannelExecutor", ThreadPoolTaskExecutor.class);