Add SQL replay checkpoint object to SQL (#868)

* Add SQL replay checkpoint object to Datastore

This will be part of the asynchronous commit-log replay to SQL. Whenever
we successfully export commits up to a particular time, we should
persist that time so we don't replay the same commits again (it is not
idempotent)

* Move SqlReplayCheckpoint from DS to SQL

* Responses to CR

db/src/main/resources/sql/flyway/V74__sql_replay_checkpoint.sql

@@ -0,0 +1,19 @@
+-- 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 "SqlReplayCheckpoint" (
+  revision_id int8 not null,
+  last_replay_time timestamptz not null,
+  PRIMARY KEY(revision_id)
+);