Some miscellaneous fixes

Related to https://build.spring.io/browse/INT-MASTERSPRING40-JOB1-1271

* Fix race condition with subscription in the `FluxMessageChannel`:
rely on the `doOnRequest()` to start producing from upstream publishers
* Remove `SourcePollingChannelAdapterTests` since
`Class.getDeclaredConstructor().newInstance()` doesn't rethrow an exception as is
but wrap it into the `InvocationTargetException`.
This is probably the main reason to deprecate a regular `Class.newInstance()`
* Use `LettuceConnectionFactory.setEagerInitialization(true)` in the `RedisAvailableRule`
to avoid possible dead lock with lazy init
* Adjust Redis tests for new `RedisAvailableRule` behavior

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

@@ -94,11 +94,10 @@ public class FluxMessageChannel extends AbstractMessageChannel
 	@Override
 	public void subscribe(Subscriber<? super Message<?>> subscriber) {
 		this.sink.asFlux()
+				.doOnRequest((r) -> this.subscribedSignal.tryEmitNext(true))
 				.doFinally((s) -> this.subscribedSignal.tryEmitNext(this.sink.currentSubscriberCount() > 0))
 				.share()
 				.subscribe(subscriber);
-
-		this.subscribedSignal.tryEmitNext(this.sink.currentSubscriberCount() > 0);
 	}
 
 	@Override