Add loadOnlyOf method to tm() (#1162)

* Add loadOnlyOf method to tm()

In addition there's a bit of a refator of SqlReplayCheckpoint to make it
more in line with the other singletons. This method is useful for the
singleton classes where we expect at most one entity to exist, e.g.
ServerSecret.

db/src/main/resources/sql/flyway/V96__rename_sql_checkpoint_fields.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 "SqlReplayCheckpoint" RENAME revision_id TO id;