Kafka Outbound Endpoints - Add futuresChannel

Currently, the only way to block is to set `sync=true` which waits for the future.
The problem with this is it only supports one-at-a-time publication.

Add an option to send the send futures to a channel, allowing the application to
send multiple records and then wait on the futures later.

Also fix long-running test.

* Add docs.

spring-integration-kafka/src/test/java/org/springframework/integration/kafka/config/xml/KafkaOutboundAdapterParserTests.java

@@ -22,10 +22,13 @@ import static org.mockito.BDDMockito.given;
 import static org.mockito.Mockito.mock;
 
 import java.time.Duration;
+import java.util.HashMap;
+import java.util.Map;
 import java.util.concurrent.Executors;
 import java.util.concurrent.TimeoutException;
 
 import org.apache.kafka.clients.producer.MockProducer;
+import org.apache.kafka.clients.producer.ProducerConfig;
 import org.apache.kafka.common.serialization.IntegerSerializer;
 import org.apache.kafka.common.serialization.StringSerializer;
 import org.junit.jupiter.api.Test;
@@ -109,12 +112,16 @@ class KafkaOutboundAdapterParserTests {
 		@SuppressWarnings("unchecked")
 		ProducerFactory<Integer, String> pf = mock(ProducerFactory.class);
 		given(pf.createProducer()).willReturn(mockProducer);
+		Map<String, Object> props = new HashMap<>();
+		props.put(ProducerConfig.DELIVERY_TIMEOUT_MS_CONFIG, 10);
+		given(pf.getConfigurationProperties()).willReturn(props);
 		KafkaTemplate<Integer, String> template = new KafkaTemplate<>(pf);
 		KafkaProducerMessageHandler<Integer, String> handler = new KafkaProducerMessageHandler<>(template);
 		handler.setBeanFactory(mock(BeanFactory.class));
 		handler.afterPropertiesSet();
 
 		handler.setSync(true);
+		handler.setSendTimeout(10);
 		handler.setTopicExpression(new LiteralExpression("foo"));
 
 		Executors.newSingleThreadExecutor()

spring-integration-kafka/src/test/java/org/springframework/integration/kafka/dsl/KafkaDslTests.java

@@ -23,6 +23,7 @@ import java.util.Collection;
 import java.util.Collections;
 import java.util.Map;
 import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.Future;
 import java.util.concurrent.TimeUnit;
 import java.util.stream.Stream;
 
@@ -49,6 +50,7 @@ import org.springframework.integration.kafka.channel.PollableKafkaChannel;
 import org.springframework.integration.kafka.inbound.KafkaMessageDrivenChannelAdapter;
 import org.springframework.integration.kafka.inbound.KafkaMessageSource;
 import org.springframework.integration.kafka.outbound.KafkaProducerMessageHandler;
+import org.springframework.integration.kafka.support.KafkaIntegrationHeaders;
 import org.springframework.integration.kafka.support.RawRecordHeaderErrorMessageStrategy;
 import org.springframework.integration.support.MessageBuilder;
 import org.springframework.integration.test.util.TestUtils;
@@ -137,6 +139,9 @@ public class KafkaDslTests {
 	@Autowired
 	private PollableChannel errorChannel;
 
+	@Autowired
+	private PollableChannel futuresChannel;
+
 	@Autowired(required = false)
 	@Qualifier("topic1ListenerContainer")
 	private MessageListenerContainer messageListenerContainer;
@@ -164,6 +169,12 @@ public class KafkaDslTests {
 
 		assertThat(TestUtils.getPropertyValue(this.kafkaProducer1, "headerMapper")).isSameAs(this.mapper);
 
+		for (int i = 0; i < 200; i++) {
+			Message<?> future = this.futuresChannel.receive(10000);
+			assertThat(future).isNotNull();
+			((Future<?>) future.getPayload()).get(10, TimeUnit.SECONDS);
+		}
+
 		for (int i = 0; i < 100; i++) {
 			Message<?> receive = this.listeningFromKafkaResults1.receive(20000);
 			assertThat(receive).isNotNull();
@@ -327,10 +338,16 @@ public class KafkaDslTests {
 			return new DefaultKafkaProducerFactory<>(props);
 		}
 
+		@Bean
+		public PollableChannel futuresChannel() {
+			return new QueueChannel();
+		}
+
 		@Bean
 		public IntegrationFlow sendToKafkaFlow() {
 			return f -> f
 					.<String>split(p -> Stream.generate(() -> p).limit(101).iterator(), null)
+					.enrichHeaders(h -> h.header(KafkaIntegrationHeaders.FUTURE_TOKEN, "foo"))
 					.publishSubscribeChannel(c -> c
 							.subscribe(sf -> sf.handle(
 									kafkaMessageHandler(producerFactory(), TEST_TOPIC1)
@@ -354,6 +371,7 @@ public class KafkaDslTests {
 
 			return Kafka
 					.outboundChannelAdapter(producerFactory)
+					.futuresChannel("futuresChannel")
 					.sync(true)
 					.messageKey(m -> m
 							.getHeaders()