Remove redundant delaySubscription from FunctionConfiguration

Related to: spring-projects/spring-integration#9362

After the fix in Spring Integration: spring-projects/spring-integration@bdcb856
we ended up in a deadlock situation with a `beginPublishingTrigger` in the `FunctionConfiguration`
used for the `delaySubscription()` on an original `Publisher`.
The `FluxMessageChannel` uses its own `delaySubscription()` until the channel has its subscribers.
Apparently the logic before was with errors, so the `FluxMessageChannel` was marked as active
even if its subscriber is not ready yet, leading to famous `Dispatcher does not have subscribers` error.
So, looks like this `beginPublishingTrigger` was introduced back in days in Spring Cloud Stream
to mitigate that situation until we really emit a `BindingCreatedEvent`.

The deadlock (and the flaw, respectively) is with the `setupBindingTrigger()` method implementation
where `FluxMessageChannel` now "really" delays a subscription to the provided `Publisher`,
therefore not triggering that `Mono.create()` fulfilment immediately.
The `BindingCreatedEvent` arrives earlier, than we have a subscriber on the channel,
but `triggerRef.get()` is `null`, so we don't `success()` it and in the end don't subscribe
to an original `Publisher` since `delaySubscription()` on it is never completed.

Since `FunctionConfiguration` fully relies on `IntegrationFlow.from(Publisher)`,
which ends up with the mentioned  `FluxMessageChannel.subscribeTo()` and its own `delaySubscription()`
(which, in turn, apparently fixed now), we don't need our own `delaySubscription()` any more.
Therefore the fix in this PR is to propose to remove `beginPublishingTrigger` logic altogether.

core/spring-cloud-stream/src/main/java/org/springframework/cloud/stream/function/FunctionConfiguration.java

@@ -40,7 +40,6 @@
 import org.reactivestreams.Publisher;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
-import reactor.core.publisher.MonoSink;
 import reactor.util.function.Tuples;
 
 import org.springframework.beans.BeansException;
@@ -65,7 +64,6 @@
 import org.springframework.cloud.function.context.message.MessageUtils;
 import org.springframework.cloud.stream.binder.BinderFactory;
 import org.springframework.cloud.stream.binder.BinderHeaders;
-import org.springframework.cloud.stream.binder.BindingCreatedEvent;
 import org.springframework.cloud.stream.binder.ConsumerProperties;
 import org.springframework.cloud.stream.binder.PartitionHandler;
 import org.springframework.cloud.stream.binder.ProducerProperties;
@@ -129,6 +127,7 @@
  * @author Byungjun You
  * @author Ivan Shapoval
  * @author Patrik Péter Süli
+ * @author Artem Bilan
  * @since 2.1
  */
 @Lazy(false)
@@ -224,8 +223,6 @@ InitializingBean supplierInitializer(FunctionCatalog functionCatalog, StreamFunc
 						functionWrapper = functionCatalog.lookup(proxyFactory.getFunctionDefinition(), contentTypes.toArray(new String[0]));
 					}
 
-					Publisher<Object> beginPublishingTrigger = setupBindingTrigger(context);
-
 					if (!functionProperties.isComposeFrom() && !functionProperties.isComposeTo()) {
 						String integrationFlowName = proxyFactory.getFunctionDefinition() + "_integrationflow";
 
@@ -239,7 +236,7 @@ InitializingBean supplierInitializer(FunctionCatalog functionCatalog, StreamFunc
 
 						if (functionWrapper != null) {
 							IntegrationFlow integrationFlow = integrationFlowFromProvidedSupplier(new PartitionAwareFunctionWrapper(functionWrapper, context, producerProperties),
-									beginPublishingTrigger, pollable, context, taskScheduler, producerProperties, outputName)
+									pollable, context, taskScheduler, producerProperties, outputName)
 									.route(Message.class, message -> {
 										if (message.getHeaders().get("spring.cloud.stream.sendto.destination") != null) {
 											String destinationName = (String) message.getHeaders().get("spring.cloud.stream.sendto.destination");
@@ -253,7 +250,7 @@ InitializingBean supplierInitializer(FunctionCatalog functionCatalog, StreamFunc
 						}
 						else {
 							IntegrationFlow integrationFlow = integrationFlowFromProvidedSupplier(new PartitionAwareFunctionWrapper(supplier, context, producerProperties),
-									beginPublishingTrigger, pollable, context, taskScheduler, producerProperties, outputName)
+									pollable, context, taskScheduler, producerProperties, outputName)
 									.channel(c -> c.direct())
 									.fluxTransform((Function<? super Flux<Message<Object>>, ? extends Publisher<Object>>) function)
 									.route(Message.class, message -> {
@@ -274,26 +271,9 @@ InitializingBean supplierInitializer(FunctionCatalog functionCatalog, StreamFunc
 		};
 	}
 
-
-	/*
-	 * Creates a publishing trigger to ensure Supplier does not begin publishing until binding is created
-	 */
-	private Publisher<Object> setupBindingTrigger(GenericApplicationContext context) {
-		AtomicReference<MonoSink<Object>> triggerRef = new AtomicReference<>();
-		Publisher<Object> beginPublishingTrigger = Mono.create(triggerRef::set);
-		context.addApplicationListener(event -> {
-			if (event instanceof BindingCreatedEvent) {
-				if (triggerRef.get() != null) {
-					triggerRef.get().success();
-				}
-			}
-		});
-		return beginPublishingTrigger;
-	}
-
 	@SuppressWarnings({ "rawtypes", "unchecked" })
 	private IntegrationFlowBuilder integrationFlowFromProvidedSupplier(Supplier<?> supplier,
-			Publisher<Object> beginPublishingTrigger, PollableBean pollable, GenericApplicationContext context,
+			PollableBean pollable, GenericApplicationContext context,
 			TaskScheduler taskScheduler, ProducerProperties producerProperties, String bindingName) {
 
 		IntegrationFlowBuilder integrationFlowBuilder;
@@ -309,8 +289,8 @@ private IntegrationFlowBuilder integrationFlowFromProvidedSupplier(Supplier<?> s
 		if (pollable == null && reactive) {
 			Publisher publisher = (Publisher) supplier.get();
 			publisher = publisher instanceof Mono
-					? ((Mono) publisher).delaySubscription(beginPublishingTrigger).map(this::wrapToMessageIfNecessary)
-					: ((Flux) publisher).delaySubscription(beginPublishingTrigger).map(this::wrapToMessageIfNecessary);
+					? ((Mono) publisher).map(this::wrapToMessageIfNecessary)
+					: ((Flux) publisher).map(this::wrapToMessageIfNecessary);
 
 			integrationFlowBuilder = IntegrationFlow.from(publisher);