Combine the two Lock classes into one class (#1126)

* Combine the two Lock classes into one class

This allows us to remove the DAO and to just treat locks the same as we
would treat any other object -- generically grabbing them from the
transaction manager.

We do not need to be concerned about the changeover between Datastore
and SQL because we assume that any such changeover will require
sufficient downtime that any currently-valid acquired locks will expire
during the downtime. Otherwise, we could get into a situation where an
action has acquired a particular lock in Datastore but not SQL.

db/src/main/resources/sql/flyway/V94__rename_lock_scope.sql

@@ -0,0 +1,15 @@
+-- Copyright 2021 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.
+
+ALTER TABLE IF EXISTS "Lock" RENAME tld TO scope;