Reduced database notification payload (saurabhnanda#43)

Because PostgreSQL limits notification payloads to 8000 bytes by
default, passing the full job record in the notification artificially
limits the job payload size.  By passing only required fields in the
notification, this limitation is removed.

src/OddJobs/Migrations.hs

@@ -32,7 +32,8 @@ createJobTableQuery tname = "CREATE TABLE " <> tname <>
 createNotificationTrigger :: TableName -> Query
 createNotificationTrigger tname = "create or replace function " <> fnName <> "() returns trigger as $$" <>
   "begin \n" <>
-  "  perform pg_notify('" <> pgEventName tname <> "', row_to_json(new)::text); \n" <>
+  "  perform pg_notify('" <> pgEventName tname <> "', \n" <>
+  "    json_build_object('id', new.id, 'run_at', new.run_at, 'locked_at', new.locked_at)::text); \n" <>
   "  return new; \n" <>
   "end; \n" <>
   "$$ language plpgsql;" <>