Harden synchronization around SockJS heartbeats

Create an explicit heartbeat task with an experiration flag so that
it can be cancelled reliably vs relying on the ScheduledFutureTask
cancel method which may return true even if the task is already
running.

Issue: SPR-14356

spring-websocket/src/test/java/org/springframework/web/socket/sockjs/transport/session/SockJsSessionTests.java

@@ -270,6 +270,7 @@ public class SockJsSessionTests extends AbstractSockJsSessionTests<TestSockJsSes
 	@Test
 	public void sendHeartbeatWhenDisabled() throws Exception {
 		this.session.disableHeartbeat();
+		this.session.setActive(true);
 		this.session.sendHeartbeat();
 
 		assertEquals(Collections.emptyList(), this.session.getSockJsFramesWritten());
@@ -292,7 +293,6 @@ public class SockJsSessionTests extends AbstractSockJsSessionTests<TestSockJsSes
 
 		this.session.cancelHeartbeat();
 
-		verify(task).isCancelled();
 		verify(task).cancel(false);
 		verifyNoMoreInteractions(task);
 	}