RDAP: Use IANA identifier as the registrar handle

According to the ICAAN operation profile:

2.1.7. Registries MUST support lookup for entities with the registrar role within other objects using the handle (as described in 3.1.5 of RFC7482). The handle of the entity with the registrar role MUST be equal to IANA Registrar ID. The entity with the registrar role in the RDAP response MUST contain a publicIDs member to identify the IANA Registrar ID from the IANA’s Registrar ID registry. The type value of the publicID object MUST be equal to IANA Registrar ID.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=130452501

java/google/registry/model/registrar/Registrar.java

@@ -286,6 +286,7 @@ public class Registrar extends ImmutableObject implements Buildable, Jsonifiable
    * </ul>
    * @see "http://www.iana.org/assignments/registrar-ids/registrar-ids.txt"
    */
+  @Index
   Long ianaIdentifier;
 
   /** Identifier of registrar used in external billing system (e.g. Oracle). */
@@ -840,6 +841,29 @@ public class Registrar extends ImmutableObject implements Buildable, Jsonifiable
       }});
   }
 
+  /**
+   * Load registrar entities by IANA identifier range outside of a transaction.
+   *
+   * @param ianaIdentifierStart returned registrars will have an IANA id greater than or equal to
+   *        this
+   * @param ianaIdentifierAfterEnd returned registrars will have an IANA id less than this
+   * @param resultSetMaxSize the maximum number of registrar entities to be returned
+   */
+  public static Iterable<Registrar> loadByIanaIdentifierRange(
+      final Long ianaIdentifierStart,
+      final Long ianaIdentifierAfterEnd,
+      final int resultSetMaxSize) {
+    return ofy().doTransactionless(new Work<Iterable<Registrar>>() {
+      @Override
+      public Iterable<Registrar> run() {
+        return ofy().load()
+            .type(Registrar.class)
+            .filter("ianaIdentifier >=", ianaIdentifierStart)
+            .filter("ianaIdentifier <", ianaIdentifierAfterEnd)
+            .limit(resultSetMaxSize);
+      }});
+  }
+
   /** Loads all registrar entities. */
   public static Iterable<Registrar> loadAll() {
     return ofy().load().type(Registrar.class).ancestor(getCrossTldKey());