fix reactor#975 windowWhen reimplementation stops retaining windows

This commit fixes `windowWhen` leaks by changing `FluxWindowWhen`
implementation, bringing it in line with that of RxJava (drained windows
were retained until main sequence completion previously).

Added padding test of QueueDrainSubscriber using the Java Object Layout
(JOL) tool.

build.gradle

@@ -253,7 +253,8 @@ project('reactor-core') {
 	testCompile "org.hamcrest:hamcrest-library:1.3",
 			"org.testng:testng:6.8.5",
 			"org.assertj:assertj-core:$assertJVersion",
-			"org.mockito:mockito-core:$mockitoVersion"
+			"org.mockito:mockito-core:$mockitoVersion",
+			"org.openjdk.jol:jol-core:0.9"
 
 	baseline("io.projectreactor:reactor-core:$compatibleVersion") {
 	  transitive = false

reactor-core/src/main/java/reactor/core/publisher/Flux.java

@@ -7713,7 +7713,6 @@ public final <U, V> Flux<Flux<T>> windowWhen(Publisher<U> bucketOpening,
 		return onAssembly(new FluxWindowWhen<>(this,
 				bucketOpening,
 				closeSelector,
-				Queues.unbounded(Queues.XS_BUFFER_SIZE),
 				Queues.unbounded(Queues.XS_BUFFER_SIZE)));
 	}