DeReference the codebase

This change replaces all Ref objects in the code with Key objects. These are
stored in datastore as the same object (raw datastore keys), so this is not
a model change.

Our best practices doc says to use Keys not Refs because:
 * The .get() method obscures what's actually going on
   - Much harder to visually audit the code for datastore loads
   - Hard to distinguish Ref<T> get()'s from Optional get()'s and Supplier get()'s
 * Implicit ofy().load() offers much less control
   - Antipattern for ultimate goal of making Ofy injectable
   - Can't control cache use or batch loading without making ofy() explicit anyway
 * Serialization behavior is surprising and could be quite dangerous/incorrect
   - Can lead to serialization errors. If it actually worked "as intended",
     it would lead to a Ref<> on a serialized object being replaced upon
     deserialization with a stale copy of the old value, which could potentially
     break all kinds of transactional expectations
 * Having both Ref<T> and Key<T> introduces extra boilerplate everywhere
   - E.g. helper methods all need to have Ref and Key overloads, or you need to
     call .key() to get the Key<T> for every Ref<T> you want to pass in
   - Creating a Ref<T> is more cumbersome, since it doesn't have all the create()
     overloads that Key<T> has, only create(Key<T>) and create(Entity) - no way to
     create directly from kind+ID/name, raw Key, websafe key string, etc.

(Note that Refs are treated specially by Objectify's @Load method and Keys are not;
we don't use that feature, but it is the one advantage Refs have over Keys.)

The direct impetus for this change is that I am trying to audit our use of memcache,
and the implicit .get() calls to datastore were making that very hard.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=131965491

java/google/registry/model/index/ForeignKeyIndex.java

@@ -23,7 +23,6 @@ import com.google.common.base.Predicate;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import com.googlecode.objectify.Key;
-import com.googlecode.objectify.Ref;
 import com.googlecode.objectify.annotation.Cache;
 import com.googlecode.objectify.annotation.Entity;
 import com.googlecode.objectify.annotation.Id;
@@ -81,9 +80,10 @@ public abstract class ForeignKeyIndex<E extends EppResource> extends BackupGroup
   /**
    * The referenced resource.
    *
-   * <p>This field holds the only reference. It is named "topReference" for historical reasons.
+   * <p>This field holds a key to the only referenced resource. It is named "topReference" for
+   * historical reasons.
    */
-  Ref<E> topReference;
+  Key<E> topReference;
 
   public String getForeignKey() {
     return foreignKey;
@@ -93,7 +93,7 @@ public abstract class ForeignKeyIndex<E extends EppResource> extends BackupGroup
     return deletionTime;
   }
 
-  public Ref<E> getReference() {
+  public Key<E> getResourceKey() {
     return topReference;
   }
 
@@ -103,7 +103,7 @@ public abstract class ForeignKeyIndex<E extends EppResource> extends BackupGroup
   public static <E extends EppResource> ForeignKeyIndex<E> create(
       E resource, DateTime deletionTime) {
     ForeignKeyIndex<E> instance = instantiate(RESOURCE_CLASS_TO_FKI_CLASS.get(resource.getClass()));
-    instance.topReference = Ref.create(resource);
+    instance.topReference = Key.create(resource);
     instance.foreignKey = resource.getForeignKey();
     instance.deletionTime = deletionTime;
     return instance;
@@ -116,7 +116,7 @@ public abstract class ForeignKeyIndex<E extends EppResource> extends BackupGroup
   }
 
   /**
-   * Loads a reference to an {@link EppResource} from the datastore by foreign key.
+   * Loads a {@link Key} to an {@link EppResource} from the datastore by foreign key.
    *
    * <p>Returns absent if no foreign key index with this foreign key was ever created, or if the
    * most recently created foreign key index was deleted before time "now". This method does not
@@ -128,10 +128,10 @@ public abstract class ForeignKeyIndex<E extends EppResource> extends BackupGroup
    * @param now the current logical time to use when checking for soft deletion of the foreign key
    *        index
    */
-  public static <E extends EppResource> Ref<E> loadAndGetReference(
+  public static <E extends EppResource> Key<E> loadAndGetKey(
       Class<E> clazz, String foreignKey, DateTime now) {
     ForeignKeyIndex<E> index = load(clazz, foreignKey, now);
-    return (index == null) ? null : index.getReference();
+    return (index == null) ? null : index.getResourceKey();
   }
 
   /**