Add support for setting removeOnCancelPolicy

This change introduces removeOnCancelPolicy on ThreadPoolTaskScheduler
and ScheduledExecutorFactoryBean and sets it to true for SockJS.
This ensures that cancelled tasks are removed immediately to avoid
the "unbounded retention of cancelled tasks" that is mentioned in
the Javadoc ScheduledThreadPoolExecutor:

"By default, such a cancelled task is not automatically removed from
the work queue until its delay elapses. While this enables further
inspection and monitoring, it may also cause unbounded retention of
cancelled tasks. To avoid this, set setRemoveOnCancelPolicy to true,
which causes tasks to be immediately removed from the work queue at
time of cancellation."

This is a backport for:
7441f23012

Issue: SPR-11918

spring-websocket/src/test/java/org/springframework/web/socket/config/MessageBrokerBeanDefinitionParserTests.java

@@ -137,6 +137,7 @@ public class MessageBrokerBeanDefinitionParserTests {
 
 		ThreadPoolTaskScheduler scheduler = (ThreadPoolTaskScheduler) defaultSockJsService.getTaskScheduler();
 		assertEquals(Runtime.getRuntime().availableProcessors(), scheduler.getScheduledThreadPoolExecutor().getCorePoolSize());
+		assertTrue(scheduler.getScheduledThreadPoolExecutor().getRemoveOnCancelPolicy());
 
 		UserSessionRegistry userSessionRegistry = this.appContext.getBean(UserSessionRegistry.class);
 		assertNotNull(userSessionRegistry);

spring-websocket/src/test/java/org/springframework/web/socket/config/annotation/WebSocketMessageBrokerConfigurationSupportTests.java

@@ -20,6 +20,7 @@ import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import java.util.concurrent.ScheduledThreadPoolExecutor;
 
 import org.junit.Before;
 import org.junit.Test;
@@ -50,6 +51,7 @@ import org.springframework.web.socket.messaging.SubProtocolHandler;
 import org.springframework.web.socket.messaging.SubProtocolWebSocketHandler;
 
 import static org.junit.Assert.*;
+import static org.junit.Assert.assertEquals;
 
 /**
  * Test fixture for
@@ -128,8 +130,9 @@ public class WebSocketMessageBrokerConfigurationSupportTests {
 		ThreadPoolTaskScheduler taskScheduler =
 				this.config.getBean("messageBrokerSockJsTaskScheduler", ThreadPoolTaskScheduler.class);
 
-		assertEquals(Runtime.getRuntime().availableProcessors(),
-				taskScheduler.getScheduledThreadPoolExecutor().getCorePoolSize());
+		ScheduledThreadPoolExecutor executor = taskScheduler.getScheduledThreadPoolExecutor();
+		assertEquals(Runtime.getRuntime().availableProcessors(), executor.getCorePoolSize());
+		assertTrue(executor.getRemoveOnCancelPolicy());
 	}