Fix 'generator object is not subscriptable' error

Some of the query functions return generators rather than lists, so we can't
index into the result. Happily we already have a copy of the results.

(think this was introduced in #7024)

changelog.d/7290.misc

@@ -0,0 +1 @@
+Move catchup of replication streams logic to worker.

synapse/replication/tcp/streams/_base.py

@@ -148,8 +148,9 @@ async def update_function(from_token, upto_token, limit):
         updates = [(row[0], row[1:]) for row in rows]
         limited = False
         if len(updates) == limit:
-            upto_token = rows[-1][0]
+            upto_token = updates[-1][0]
             limited = True
+        assert len(updates) <= limit
 
         return updates, upto_token, limited