Fix bug where read-receipts lost their timestamps (#4927)

Make sure that they are sent correctly over the replication stream.

Fixes: #4898

synapse/storage/receipts.py

@@ -301,7 +301,9 @@ class ReceiptsWorkerStore(SQLBaseStore):
                 args.append(limit)
             txn.execute(sql, args)
 
-            return txn.fetchall()
+            return (
+                r[0:5] + (json.loads(r[5]), ) for r in txn
+            )
         return self.runInteraction(
             "get_all_updated_receipts", get_all_updated_receipts_txn
         )