Store Vaadin session after each update

This helps clustering solutions know when the session attribute needs
to be replicated instead of having to always aggressively replicate
all attributes.

Resolves #7535

server/src/main/java/com/vaadin/server/VaadinSession.java

                                         + ui.getUIId(),
                                 e);
                     }
+                    // Store session after modifications have been done so that
+                    // Spring Session and possibly other implementations realize
+                    // that something has changed inside the session attribute
+                    service.storeSession(this, session);
                 }
             }
         } finally {

server/src/test/java/com/vaadin/ui/UITest.java

 import java.util.concurrent.locks.ReentrantLock;
 
 import javax.servlet.ServletConfig;
+import javax.servlet.http.HttpSession;
 
+import org.easymock.EasyMock;
 import org.junit.Assert;
 import org.junit.Test;
 import org.mockito.Mockito;
 import com.vaadin.server.VaadinServlet;
 import com.vaadin.server.VaadinServletService;
 import com.vaadin.server.VaadinSession;
+import com.vaadin.server.WrappedHttpSession;
 import com.vaadin.server.communication.PushConnection;
 import com.vaadin.shared.communication.PushMode;
 import com.vaadin.util.CurrentInstanceTest;
         VaadinServletService service = new VaadinServletService(servlet,
                 deploymentConfiguration);
         MockVaadinSession session = new MockVaadinSession(service);
+        HttpSession mockHttpSession = EasyMock.createMock(HttpSession.class);
+        WrappedHttpSession mockWrappedSession = new WrappedHttpSession(
+                mockHttpSession) {
+            @Override
+            public Object getAttribute(String name) {
+                String lockAttribute = service.getServiceName() + ".lock";
+                if (lockAttribute.equals(name)) {
+                    return session.getLockInstance();
+                } else {
+                    return super.getAttribute(name);
+                }
+            }
+        };
         session.lock();
+        session.refreshTransients(mockWrappedSession, service);
         ui.setSession(session);
         ui.doInit(Mockito.mock(VaadinRequest.class), 1, "foo");
         session.addUI(ui);