Add a to_device_stream_id column to the application_services_state table

This is for tracking the stream id that each application service has
been sent up to. In other words, there shouldn't be any need to process
stream ids below the recorded one here as the AS should have already
received them.

Note that there is no reliability built-in here. Reliability of delivery
if intended for a separate PR.

synapse/storage/schema/main/delta/65/06_msc2409_add_device_id_appservice_stream_type.sql

@@ -0,0 +1,18 @@
+/* Copyright 2021 The Matrix.org Foundation C.I.C
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+-- Add a column to track what to_device stream id that this application
+-- service has been caught up to.
+ALTER TABLE application_services_state ADD COLUMN to_device_stream_id BIGINT;