When loading current ids, sort by `stream_id` to avoid incorrect overwrite and avoid errors caused by sorting alphabetical instance name which can be `null` (#13585)

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 ([discussion](https://github.com/matrix-org/synapse/pull/13585#discussion_r951795379)). 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, notice the `instance_name` is `null` in the db, and we can't sort `NoneType` things. Another question is why do we see the `instance_name` as `null` sometimes instead of `master` in monolith mode?
```
$ 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
```

1 files changed, 11 insertions, 2 deletions

diff --git a/synapse/storage/util/id_generators.py b/synapse/storage/util/id_generators.py
index 3c13859faa..2dfe4c0b66 100644
--- a/synapse/storage/util/id_generators.py
+++ b/synapse/storage/util/id_generators.py
@@ -460,8 +460,17 @@ class MultiWriterIdGenerator(AbstractStreamIdGenerator):
                 # Cast safety: this corresponds to the types returned by the query above.
                 rows.extend(cast(Iterable[Tuple[str, int]], cur))
 
-            # Sort so that we handle rows in order for each instance.
-            rows.sort()
+            # Sort by stream_id (ascending, lowest -> highest) so that we handle
+            # rows in order for each instance because we don't want to overwrite
+            # the current_position of an instance to a lower stream ID than
+            # we're actually at.
+            def sort_by_stream_id_key_func(row: Tuple[str, int]) -> int:
+                (instance, stream_id) = row
+                # If `stream_id` is ever `None`, we will see a `TypeError: '<'
+                # not supported between instances of 'NoneType' and 'X'` error.
+                return stream_id
+
+            rows.sort(key=sort_by_stream_id_key_func)
 
             with self._lock:
                 for (