Fixing issue with Push stopping working in some circumstances (#11791)

* Fixing issue with Push stopping working in some circumstances

If new request is attempted when resynchronization is ongoing, the Push will stop working. This patch fixes the issue by aborting handleJson if resynch is already ongoing. 

This PR supercedes https://github.com/vaadin/framework/pull/11786

Fixes #11702, #7719

* Call onResynchronize() in MessageHandler

* Optimizing

client/src/main/java/com/vaadin/client/communication/MessageHandler.java

     private int lastSeenServerSyncId = UNDEFINED_SYNC_ID;
 
     private ApplicationConnection connection;
+    private boolean resyncInProgress;
 
     /**
      * Data structure holding information about pending UIDL messages.
     protected void handleJSON(final ValueMap json) {
         final int serverId = getServerId(json);
 
-        if (isResynchronize(json) && !isNextExpectedMessage(serverId)) {
+        boolean hasResynchronize = isResynchronize(json);
+
+        if (!hasResynchronize && resyncInProgress) {
+            Logger.getLogger(MessageHandler.class.getName())
+                .warning("Dropping the response of a request before a resync request.");
+            return;
+        }
+
+        resyncInProgress = false;
+
+        if (hasResynchronize && !isNextExpectedMessage(serverId)) {
             // Resynchronize request. We must remove any old pending
             // messages and ensure this is handled next. Otherwise we
             // would keep waiting for an older message forever (if this
             int serverNextExpected = json
                     .getInt(ApplicationConstants.CLIENT_TO_SERVER_ID);
             getMessageSender().setClientToServerMessageId(serverNextExpected,
-                    isResynchronize(json));
+                    hasResynchronize);
         }
 
         if (serverId != -1) {
         }
     }-*/;
 
+    public void onResynchronize() {
+        resyncInProgress = true;
+    }
 }

client/src/main/java/com/vaadin/client/communication/MessageSender.java

      * state from the server
      */
     public void resynchronize() {
+        getMessageHandler().onResynchronize();
         getLogger().info("Resynchronizing from server");
         JsonObject resyncParam = Json.createObject();
         resyncParam.put(ApplicationConstants.RESYNCHRONIZE_ID, true);