Load foreign keys more efficiently for xml marshalling.

Before this CL, each contact and host was independently
loaded via the ReferenceUnion adapter. Since fields are
processed serially by JAXB, this means worst-case there
were 17 loads, best case 3 (the 3 required contacts) and
usual case 5-6 (some hosts). This CL reduces that to 1
datastore roundtrip in all cases.

A side effect of this CL is the further hollowing-out of
ReferenceUnion, since it no longer is involved in
marshalling at all.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=123712842

java/google/registry/model/domain/ForeignKeyedDesignatedContact.java

@@ -0,0 +1,41 @@
+// Copyright 2016 The Domain Registry Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package google.registry.model.domain;
+
+import google.registry.model.ImmutableObject;
+
+import javax.xml.bind.annotation.XmlAttribute;
+import javax.xml.bind.annotation.XmlValue;
+
+/**
+ * EPP-compatible version of XML type for contact identifiers associated with a domain, which can
+ * be converted to a storable {@link DesignatedContact}.
+ *
+ * @see "http://tools.ietf.org/html/rfc5731#section-2.2"
+ */
+class ForeignKeyedDesignatedContact extends ImmutableObject {
+  @XmlAttribute(required = true)
+  DesignatedContact.Type type;
+
+  @XmlValue
+  String contactId;
+
+  static ForeignKeyedDesignatedContact create(DesignatedContact.Type type, String contactId) {
+    ForeignKeyedDesignatedContact instance = new ForeignKeyedDesignatedContact();
+    instance.type = type;
+    instance.contactId = contactId;
+    return instance;
+  }
+}