Change sendMessage access modifier to protected. (#9622)

This allows extending AtmospherePushConnection, and then doing overriding sendMessage. One use case is to correlate the IP address and the message in the logs.

server/src/main/java/com/vaadin/server/communication/AtmospherePushConnection.java

      * @param message
      *            The message to send
      */
-    void sendMessage(String message) {
+    protected void sendMessage(String message) {
         assert (isConnected());
         // "Broadcast" the changes to the single client only
         outgoingMessage = getResource().getBroadcaster().broadcast(message,