Add the schema and DAO for Locks in CloudSQL (#462)

* Add the schema and DAO for Locks in CloudSQL

* Addresses some comments

* Change number on flyway file

* Small changes

* More small changes

* Use checkArgumentNotNull instead of checkNotNull

* Address comments

* fix javadocs

* update persistence

db/src/main/resources/sql/flyway/V18__create_lock.sql

@@ -0,0 +1,22 @@
+ -- Copyright 2020 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 table "Lock" (
+       resource_name text not null,
+        tld text not null,
+        acquired_time timestamptz not null,
+        expiration_time timestamptz not null,
+        request_log_id text not null,
+        primary key (resource_name, tld)
+    );