Test that marshaling a domain only does one datastore fetch.

This is true even though the domain has three fields (a contact,
a host, and the registrant) whose foreign keys need to be loaded.

This CL also adds the generic ability to do these sort of tests
elsewhere in the code, by instrumenting the datastore instance
used by Objectify to store static counts of method calls.

TESTED=patched in a rollback of [] and confirmed that the
test failed because there were three reads.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=123885768

java/google/registry/model/ofy/ObjectifyService.java

@@ -20,6 +20,8 @@ import static com.google.common.base.Predicates.not;
 import static com.googlecode.objectify.ObjectifyService.factory;
 import static google.registry.util.TypeUtils.hasAnnotation;
 
+import com.google.appengine.api.datastore.AsyncDatastoreService;
+import com.google.appengine.api.datastore.DatastoreServiceConfig;
 import com.google.appengine.api.datastore.DatastoreServiceFactory;
 import com.google.common.annotations.VisibleForTesting;
 import com.google.common.collect.Iterables;
@@ -99,6 +101,16 @@ public class ObjectifyService {
       @Override
       public Objectify begin() {
         return new SessionKeyExposingObjectify(this);
+      }
+
+      @Override
+      protected AsyncDatastoreService createRawAsyncDatastoreService(DatastoreServiceConfig cfg) {
+        // In the unit test environment, wrap the datastore service in a proxy that can be used to
+        // examine the number of requests sent to datastore.
+        AsyncDatastoreService service = super.createRawAsyncDatastoreService(cfg);
+        return RegistryEnvironment.get().equals(RegistryEnvironment.UNITTEST)
+            ? new RequestCountingAsyncDatastoreService(service)
+            : service;
       }});
 
     // Translators must be registered before any entities can be registered.