Log the first entry into our code per request

Trying to debug the 20s delay in requests, it would help to know if the delay
happens before or after our code is called.

Right now all we know is that the delay happens before our first loggin line,
which is in RequestAuthenticator.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=182211285

java/google/registry/module/backend/BackendServlet.java

@@ -62,6 +62,7 @@ public final class BackendServlet extends HttpServlet {
 
   @Override
   public void service(HttpServletRequest req, HttpServletResponse rsp) throws IOException {
+    logger.info("Received backend request");
     requestHandler.handleRequest(req, rsp);
   }
 }