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/schema/db-schema.sql.generated

@@ -695,9 +695,9 @@
     );
 
     create table "SqlReplayCheckpoint" (
-       revision_id int8 not null,
-        last_replay_time timestamptz,
-        primary key (revision_id)
+       id int8 not null,
+        last_replay_time timestamptz not null,
+        primary key (id)
     );
 
     create table "Tld" (