Ensure portdb selects rows with negative rowids

The `backward_select` query uses the `backward_chunk` variable as an
inclusive upper bound on the rowids it selects. It is initially 0 (see
`setup_table`). It is then set to

```
backward_chunk = min(row[0] for row in brows) - 1
```

where `brows` is the result of running the `backwards_select` query.
For this to make sense, we need to ensure that `backwards_select` picks
rows in descending order. Otherwise we'll jump right to the bottom of
the rowids, pick out the lowest batch only and discard everything we
skipped over. This is a Bad Thing.

I've tested this locally with the reproduction case reported in #13191.
Without the patch, I could reproduce the reported failure; with the
patch, the portdb script completes successfully.

synapse/_scripts/synapse_port_db.py

@@ -418,12 +418,15 @@ async def handle_table(
             self.progress.update(table, table_size)  # Mark table as done
             return
 
+        # We sweep over rowids in two directions: one forwards (rowids 1, 2, 3, ...)
+        # and another backwards (rowids 0, -1, -2, ...).
         forward_select = (
             "SELECT rowid, * FROM %s WHERE rowid >= ? ORDER BY rowid LIMIT ?" % (table,)
         )
 
         backward_select = (
-            "SELECT rowid, * FROM %s WHERE rowid <= ? ORDER BY rowid LIMIT ?" % (table,)
+            "SELECT rowid, * FROM %s WHERE rowid <= ? ORDER BY rowid DESC LIMIT ?"
+            % (table,)
         )
 
         do_forward = [True]