Add a registrarId index to RegistryLock (#312)

* Add a registrarId index to RegistryLock

* Merge remote-tracking branch 'origin/master' into getByRegistrar

* Responses to CR

db/src/main/resources/sql/flyway/V8__registry_lock_registrar_index.sql

@@ -0,0 +1,16 @@
+-- Copyright 2019 The Nomulus 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.
+
+create index if not exists idx_registry_lock_registrar_id ON "RegistryLock"
+   using btree (registrar_id);