Refactor getting replication updates from database. (#7636)

The aim here is to make it easier to reason about when streams are limited and when they're not, by moving the logic into the database functions themselves. This should mean we can kill of `db_query_to_update_function` function.

synapse/push/pusherpool.py

@@ -215,11 +215,9 @@ class PusherPool:
         try:
             # Need to subtract 1 from the minimum because the lower bound here
             # is not inclusive
-            updated_receipts = yield self.store.get_all_updated_receipts(
+            users_affected = yield self.store.get_users_sent_receipts_between(
                 min_stream_id - 1, max_stream_id
             )
-            # This returns a tuple, user_id is at index 3
-            users_affected = {r[3] for r in updated_receipts}
 
             for u in users_affected:
                 if u in self.pushers: