Use Flux.handle() instead of doOnNext() in FluxMC

For better back-pressure handling do not perform active operations
directly on the current `Flux` (e.g. via `doOnNext()`).
It's better to postpone such a handling to back-pressure ready operators
Like in this `FluxMessageChannel` case, the `.handle()` operator is
wrapping the "hard" `send()` operation.

Also include `.errorStrategyContinue()` do not stop during message
processing

spring-integration-core/src/main/java/org/springframework/integration/channel/FluxMessageChannel.java

@@ -45,7 +45,7 @@ public class FluxMessageChannel extends AbstractMessageChannel
 
 	private final List<Subscriber<? super Message<?>>> subscribers = new ArrayList<>();
 
-	private final Map<Publisher<Message<?>>, ConnectableFlux<Message<?>>> publishers = new ConcurrentHashMap<>();
+	private final Map<Publisher<Message<?>>, ConnectableFlux<?>> publishers = new ConcurrentHashMap<>();
 
 	private final Flux<Message<?>> flux;
 
@@ -79,10 +79,11 @@ public class FluxMessageChannel extends AbstractMessageChannel
 
 	@Override
 	public void subscribeTo(Publisher<Message<?>> publisher) {
-		ConnectableFlux<Message<?>> connectableFlux =
+		ConnectableFlux<?> connectableFlux =
 				Flux.from(publisher)
+						.handle((message, sink) -> sink.next(send(message)))
+						.errorStrategyContinue()
 						.doOnComplete(() -> this.publishers.remove(publisher))
-						.doOnNext(this::send)
 						.publish();
 
 		this.publishers.put(publisher, connectableFlux);