Remove nearly all uses of ReferenceUnion

ReferenceUnion is a hack to work around the mismatch between how
we store references (by roid) and how they are represented in EPP
(by foreign key). If it ever needed to exist (not entirely clear...)
it should have remained tightly scoped within the domain commands
and resources. Instead it has leaked everywhere in the project,
causing lots of boilerplate. This CL hides all of that behind
standard Refs, and should be followed by work to remove ReferenceUnion
completely.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=122424416

java/google/registry/flows/async/DeleteContactResourceAction.java

@@ -16,9 +16,10 @@ package google.registry.flows.async;
 
 import static google.registry.flows.ResourceFlowUtils.handlePendingTransferOnDelete;
 
+import com.googlecode.objectify.Ref;
+
 import google.registry.model.contact.ContactResource;
 import google.registry.model.domain.DomainBase;
-import google.registry.model.domain.ReferenceUnion;
 import google.registry.model.reporting.HistoryEntry;
 import google.registry.model.reporting.HistoryEntry.Type;
 import google.registry.request.Action;
@@ -48,7 +49,7 @@ public class DeleteContactResourceAction extends DeleteEppResourceAction<Contact
 
     @Override
     protected boolean isLinked(
-        DomainBase domain, ReferenceUnion<ContactResource> targetResourceRef) {
+        DomainBase domain, Ref<ContactResource> targetResourceRef) {
       return domain.getReferencedContacts().contains(targetResourceRef);
     }
   }

java/google/registry/flows/async/DeleteEppResourceAction.java

@@ -41,7 +41,6 @@ import google.registry.mapreduce.inputs.NullInput;
 import google.registry.model.EppResource;
 import google.registry.model.annotations.ExternalMessagingName;
 import google.registry.model.domain.DomainBase;
-import google.registry.model.domain.ReferenceUnion;
 import google.registry.model.eppcommon.StatusValue;
 import google.registry.model.poll.PollMessage;
 import google.registry.model.reporting.HistoryEntry;
@@ -110,7 +109,7 @@ public abstract class DeleteEppResourceAction<T extends EppResource> implements
         "Resource %s is already deleted.", resource.getForeignKey());
     checkState(
         resource.getStatusValues().contains(StatusValue.PENDING_DELETE),
-        "Resource %s is not set as PENDING_DELETE", resource.getForeignKey());    
+        "Resource %s is not set as PENDING_DELETE", resource.getForeignKey());
     mapper.setTargetResource(resourceKey);
     reducer.setClient(requestingClientId, isSuperuser);
     logger.infofmt("Executing Delete EPP resource mapreduce for %s", resourceKey);
@@ -147,7 +146,7 @@ public abstract class DeleteEppResourceAction<T extends EppResource> implements
     }
 
     /** Determine whether the target resource is a linked resource on the domain. */
-    protected abstract boolean isLinked(DomainBase domain, ReferenceUnion<T> targetResourceRef);
+    protected abstract boolean isLinked(DomainBase domain, Ref<T> targetResourceRef);
 
     @Override
     public void map(DomainBase domain) {
@@ -159,12 +158,12 @@ public abstract class DeleteEppResourceAction<T extends EppResource> implements
         emit(targetEppResourceKey, false);
         return;
       }
-      // The ReferenceUnion can't be a field on the Mapper, because when a Ref<?> is serialized
-      // (required for each MapShardTask), it uses the DeadRef version, which contains the Ref's
-      // value, which isn't serializable. Thankfully, this isn't expensive.
+      // The Ref can't be a field on the Mapper, because when a Ref<?> is serialized (required for
+      // each MapShardTask), it uses the DeadRef version, which contains the Ref's value, which
+      // isn't serializable. Thankfully, this isn't expensive.
       // See: https://github.com/objectify/objectify/blob/master/src/main/java/com/googlecode/objectify/impl/ref/DeadRef.java
       if (isActive(domain, targetResourceUpdateTimestamp)
-          && isLinked(domain, ReferenceUnion.create(Ref.create(targetEppResourceKey)))) {
+          && isLinked(domain, Ref.create(targetEppResourceKey))) {
         emit(targetEppResourceKey, true);
       }
     }

java/google/registry/flows/async/DeleteHostResourceAction.java

@@ -16,9 +16,10 @@ package google.registry.flows.async;
 
 import static google.registry.model.ofy.ObjectifyService.ofy;
 
+import com.googlecode.objectify.Ref;
+
 import google.registry.dns.DnsQueue;
 import google.registry.model.domain.DomainBase;
-import google.registry.model.domain.ReferenceUnion;
 import google.registry.model.host.HostResource;
 import google.registry.model.reporting.HistoryEntry;
 import google.registry.model.reporting.HistoryEntry.Type;
@@ -48,8 +49,7 @@ public class DeleteHostResourceAction extends DeleteEppResourceAction<HostResour
     private static final long serialVersionUID = 1941092742903217194L;
 
     @Override
-    protected boolean isLinked(
-        DomainBase domain, ReferenceUnion<HostResource> targetResourceRef) {
+    protected boolean isLinked(DomainBase domain, Ref<HostResource> targetResourceRef) {
       return domain.getNameservers().contains(targetResourceRef);
     }
   }

java/google/registry/flows/async/DnsRefreshForHostRenameAction.java

@@ -30,7 +30,6 @@ import google.registry.mapreduce.MapreduceAction;
 import google.registry.mapreduce.MapreduceRunner;
 import google.registry.mapreduce.inputs.EppResourceInputs;
 import google.registry.model.domain.DomainResource;
-import google.registry.model.domain.ReferenceUnion;
 import google.registry.model.host.HostResource;
 import google.registry.request.Action;
 import google.registry.request.HttpException.BadRequestException;
@@ -98,7 +97,7 @@ public class DnsRefreshForHostRenameAction implements MapreduceAction {
     @Override
     public final void map(DomainResource domain) {
       if (isActive(domain, hostUpdateTime)
-          && domain.getNameservers().contains(ReferenceUnion.create(Ref.create(targetHostKey)))) {
+          && domain.getNameservers().contains(Ref.create(targetHostKey))) {
         try {
           dnsQueue.addDomainRefreshTask(domain.getFullyQualifiedDomainName());
           logger.infofmt("Enqueued refresh for domain %s", domain.getFullyQualifiedDomainName());