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/flows/host/HostDeleteFlow.java

@@ -21,7 +21,6 @@ import com.google.common.base.Predicate;
 import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.Iterables;
 import com.googlecode.objectify.Key;
-import com.googlecode.objectify.Ref;
 import google.registry.config.RegistryEnvironment;
 import google.registry.flows.EppException;
 import google.registry.flows.ResourceAsyncDeleteFlow;
@@ -51,7 +50,7 @@ public class HostDeleteFlow extends ResourceAsyncDeleteFlow<HostResource, Builde
   @Inject HostDeleteFlow() {}
 
   @Override
-  protected boolean isLinkedForFailfast(final Ref<HostResource> ref) {
+  protected boolean isLinkedForFailfast(final Key<HostResource> key) {
     // Query for the first few linked domains, and if found, actually load them. The query is
     // eventually consistent and so might be very stale, but the direct load will not be stale,
     // just non-transactional. If we find at least one actual reference then we can reliably
@@ -59,11 +58,11 @@ public class HostDeleteFlow extends ResourceAsyncDeleteFlow<HostResource, Builde
     return Iterables.any(
         ofy().load().keys(
             queryDomainsUsingResource(
-                  HostResource.class, ref, now, FAILFAST_CHECK_COUNT)).values(),
+                  HostResource.class, key, now, FAILFAST_CHECK_COUNT)).values(),
         new Predicate<DomainBase>() {
             @Override
             public boolean apply(DomainBase domain) {
-              return domain.getNameservers().contains(ref);
+              return domain.getNameservers().contains(key);
             }});
   }