Make AppserviceHandler stream events from database

This is for two reasons:

1. Suppresses duplicates correctly, as the notifier doesn't do any
   duplicate suppression.
2. Makes it easier to connect the AppserviceHandler to the replication
   stream.

synapse/storage/schema/delta/34/appservice_stream.sql

@@ -0,0 +1,23 @@
+/* Copyright 2016 OpenMarket Ltd
+ *
+ * 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.
+ */
+
+CREATE TABLE IF NOT EXISTS appservice_stream_position(
+    Lock CHAR(1) NOT NULL DEFAULT 'X' UNIQUE,  -- Makes sure this table only has one row.
+    stream_ordering BIGINT,
+    CHECK (Lock='X')
+);
+
+INSERT INTO appservice_stream_position (stream_ordering)
+    SELECT COALESCE(MAX(stream_ordering), 0) FROM events;