Synchronize request init in AbstractHttpSockJsSession

Although unlikely in practice (but not impossible), the SockJS
integration tests write a message while the request is initializing.
This change adds synchronization around request intiailization
for the SockJS HTTP sesion.

This is a backport of:
59e02e63c4

Issue: SPR-11916

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

@@ -201,6 +201,7 @@ public abstract class AbstractHttpSockJsSession extends AbstractSockJsSession {
 		this.frameFormat = frameFormat;
 		this.asyncRequestControl = request.getAsyncRequestControl(response);
 
+		synchronized (this.responseLock) {
 			try {
 				// Let "our" handler know before sending the open frame to the remote handler
 				delegateConnectionEstablished();
@@ -215,6 +216,7 @@ public abstract class AbstractHttpSockJsSession extends AbstractSockJsSession {
 				throw new SockJsTransportFailureException("Failed to open session", getId(), ex);
 			}
 		}
+	}
 
 	protected void writePrelude(ServerHttpRequest request, ServerHttpResponse response) throws IOException {
 	}