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/schema/db-schema.sql.generated

@@ -655,6 +655,12 @@
         primary key (id)
     );
 
+    create table "SqlReplayCheckpoint" (
+       revision_id int8 not null,
+        last_replay_time timestamptz,
+        primary key (revision_id)
+    );
+
     create table "Tld" (
        tld_name text not null,
         add_grace_period_length interval not null,