INT-4280: Add taskScheduler() to Java DSL

JIRA: https://jira.spring.io/browse/INT-4280

To let end-user easily to inject custom `TaskScheduler` to the endpoint
add `ConsumerEndpointSpec.taskScheduler()` option

* Address PR comments

spring-integration-core/src/test/java/org/springframework/integration/dsl/flows/IntegrationFlowTests.java

@@ -26,8 +26,11 @@ import static org.junit.Assert.assertThat;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
 
+import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicReference;
 
 import org.aopalliance.aop.Advice;
 import org.aopalliance.intercept.MethodInterceptor;
@@ -427,6 +430,28 @@ public class IntegrationFlowTests {
 		assertEquals("foo", this.errorRecovererFlowGateway.testIt("foo"));
 	}
 
+	@Autowired
+	private MessageChannel dedicatedQueueChannel;
+
+	@Autowired
+	private SubscribableChannel dedicatedResults;
+
+	@Test
+	public void testDedicatedPollingThreadFlow() throws InterruptedException {
+		AtomicReference<String> threadNameReference = new AtomicReference<>();
+		CountDownLatch resultLatch = new CountDownLatch(1);
+		this.dedicatedResults.subscribe(m -> {
+			threadNameReference.set(Thread.currentThread().getName());
+			resultLatch.countDown();
+		});
+
+		this.dedicatedQueueChannel.send(new GenericMessage<>("foo"));
+
+		assertTrue(resultLatch.await(10, TimeUnit.SECONDS));
+
+		assertEquals("dedicatedTaskScheduler-1", threadNameReference.get());
+	}
+
 	@MessagingGateway
 	public interface ControlBusGateway {
 
@@ -721,6 +746,22 @@ public class IntegrationFlowTests {
 
 		}
 
+		@Bean
+		public IntegrationFlow dedicatedPollingThreadFlow() {
+			return IntegrationFlows.from(MessageChannels.queue("dedicatedQueueChannel"))
+					.bridge(e -> e
+							.poller(Pollers.fixedDelay(0).receiveTimeout(-1))
+							.taskScheduler(dedicatedTaskScheduler()))
+					.channel("dedicatedResults")
+					.get();
+		}
+
+
+		@Bean
+		public TaskScheduler dedicatedTaskScheduler() {
+			return new ThreadPoolTaskScheduler();
+		}
+
 	}
 
 	@MessagingGateway