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/DomainCommand.java

@@ -510,20 +510,6 @@ public class DomainCommand {
           }}));
   }
 
-  /**
-   * EPP-inputable version of XML type for contact identifiers associated with a domain, which can
-   * be converted to a storable (and EPP-outputable) {@link DesignatedContact}.
-   *
-   * @see "http://tools.ietf.org/html/rfc5731#section-2.2"
-   */
-  static class ForeignKeyedDesignatedContact extends ImmutableObject {
-    @XmlAttribute(required = true)
-    DesignatedContact.Type type;
-
-    @XmlValue
-    String contactId;
-  }
-
   /** Exception to throw when referenced objects don't exist. */
   public static class InvalidReferencesException extends Exception {
     private final ImmutableSet<String> foreignKeys;