Support user destinations with multiple app servers

This change adds support for broadcasting messages with unresolved
user destinations so that other servers can try to resolve it.
That enables sending messages to users who may be connected to a
different server.

Issue: SPR-11620

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

@@ -275,6 +275,12 @@ public class MessageBrokerBeanDefinitionParserTests {
 			// expected
 		}
 
+		UserDestinationMessageHandler userDestHandler = this.appContext.getBean(UserDestinationMessageHandler.class);
+		assertEquals("/topic/unresolved", userDestHandler.getUserDestinationBroadcast());
+		assertNotNull(messageBroker.getSystemSubscriptions());
+		assertSame(userDestHandler, messageBroker.getSystemSubscriptions().get("/topic/unresolved"));
+
+
 		String name = "webSocketMessageBrokerStats";
 		WebSocketMessageBrokerStats stats = this.appContext.getBean(name, WebSocketMessageBrokerStats.class);
 		String actual = stats.toString();