Fix Datastore "count" queries (#1201)

* Fix Datastore "count" queries

The objectify "count()" method doesn't work for result sets larger than 1000
elements, use the original trick from "count domains" that fetches the keys
and counts them.

* Added an SO link

core/src/main/java/google/registry/model/ofy/DatastoreTransactionManager.java

@@ -480,7 +480,13 @@ public class DatastoreTransactionManager implements TransactionManager {
 
     @Override
     public long count() {
-      return buildQuery().count();
+      // Objectify provides a count() function, but unfortunately that doesn't work if there are
+      // more than 1000 (the default response page size?) entries in the result set.  We also use
+      // chunkAll() here as it provides a nice performance boost.
+      //
+      // There is some information on this issue on SO, see:
+      // https://stackoverflow.com/questions/751124/how-does-one-get-a-count-of-rows-in-a-datastore-model-in-google-app-engine
+      return Iterables.size(buildQuery().chunkAll().keys());
     }
 
     @Override