…nstance name

…lower than we're actually at

See #13585 (comment)

When loading current ids, sort by stream ID so that we don't want to overwrite
the `current_position` of an instance to a lower stream ID than we're actually at.
Previously, it sorted alphabetically by instance name which can be `null` and
throw errors but more importantly, accomplishes nothing.

Fixes the following startup error which is why I started looking into this area:

```
$ poetry run synapse_homeserver --config-path homeserver.yaml
****************************************************************
 Error during initialisation:
    '<' not supported between instances of 'NoneType' and 'str'
 There may be more information in the logs.
****************************************************************
```

Somehow my database ended up looking like the following:
```
$ psql synapse
synapse=# SELECT * FROM stream_positions;
   stream_name   | instance_name | stream_id
-----------------+---------------+-----------
 account_data    | master        |      1242
 events          | master        |      1787
 to_device       | master        |        58
 presence_stream | master        |    485638
 receipts        | master        |       341
 backfill        | master        |   -139106
(6 rows)
synapse=# SELECT instance_name, stream_id FROM receipts_linearized;
 instance_name | stream_id
---------------+-----------
               |       211
               |         3
               |         4
               |       212
               |       213
               |       224
               |       228
               |       164
               |       313
               |       253
               |        38
               |       321
               |       324
               |       189
               |       192
               |       193
               |       194
               |       195
               |       197
               |       198
               |       275
               |        79
               |       339
               |       340
               |        82
               |       341
               |        84
               |        85
               |        91
               |       119
```

… intent

See #13585 (comment)