Remove synchronized block around WebSocketSession.send

Since we now wrap the WebSocketSession with a concurrent decorator, the
synchronized keyword around message sending needed to be removed.

Issue: SPR-11586

spring-websocket/src/main/java/org/springframework/web/socket/handler/ConcurrentWebSocketSessionDecorator.java

@@ -118,6 +118,11 @@ public class ConcurrentWebSocketSessionDecorator extends WebSocketSessionDecorat
 	}
 
 	private void checkSessionLimits() throws IOException {
+		if (logger.isDebugEnabled()) {
+			logger.debug("Another send already in progress, session id '" + getId() + "'" +
+					", in-progress send time " + getInProgressSendTime() + " (ms), " +
+					", buffer size " + this.bufferSize + " bytes");
+		}
 		if (getInProgressSendTime() > this.sendTimeLimit) {
 			logError("A message could not be sent due to a timeout");
 			getDelegate().close();

spring-websocket/src/main/java/org/springframework/web/socket/messaging/StompSubProtocolHandler.java

@@ -198,11 +198,9 @@ public class StompSubProtocolHandler implements SubProtocolHandler {
 		try {
 			message = MessageBuilder.withPayload(message.getPayload()).setHeaders(headers).build();
 			byte[] bytes = this.stompEncoder.encode((Message<byte[]>) message);
+			TextMessage textMessage = new TextMessage(bytes);
 
-			synchronized(session) {
-				session.sendMessage(new TextMessage(bytes));
-			}
-
+			session.sendMessage(textMessage);
 		}
 		catch (Throwable ex) {
 			sendErrorMessage(session, ex);

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/transport/session/WebSocketServerSockJsSession.java

@@ -174,7 +174,7 @@ public class WebSocketServerSockJsSession extends AbstractSockJsSession implemen
 
 	@Override
 	protected void disconnect(CloseStatus status) throws IOException {
-		if (this.webSocketSession != null) {
+		if (isActive()) {
 			this.webSocketSession.close(status);
 		}
 	}