Reorganise the database schema directories (#9932)

The hope here is that by moving all the schema files into synapse/storage/schema, it gets a bit easier for newcomers to navigate.

It certainly got easier for me to write a helpful README. There's more to do on that front, but I'll follow up with other PRs for that.

synapse/storage/schema/main/delta/27/ts.py

@@ -0,0 +1,57 @@
+# Copyright 2015, 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.
+import json
+import logging
+
+from synapse.storage.prepare_database import get_statements
+
+logger = logging.getLogger(__name__)
+
+
+ALTER_TABLE = (
+    "ALTER TABLE events ADD COLUMN origin_server_ts BIGINT;"
+    "CREATE INDEX events_ts ON events(origin_server_ts, stream_ordering);"
+)
+
+
+def run_create(cur, database_engine, *args, **kwargs):
+    for statement in get_statements(ALTER_TABLE.splitlines()):
+        cur.execute(statement)
+
+    cur.execute("SELECT MIN(stream_ordering) FROM events")
+    rows = cur.fetchall()
+    min_stream_id = rows[0][0]
+
+    cur.execute("SELECT MAX(stream_ordering) FROM events")
+    rows = cur.fetchall()
+    max_stream_id = rows[0][0]
+
+    if min_stream_id is not None and max_stream_id is not None:
+        progress = {
+            "target_min_stream_id_inclusive": min_stream_id,
+            "max_stream_id_exclusive": max_stream_id + 1,
+            "rows_inserted": 0,
+        }
+        progress_json = json.dumps(progress)
+
+        sql = (
+            "INSERT into background_updates (update_name, progress_json)"
+            " VALUES (?, ?)"
+        )
+
+        cur.execute(sql, ("event_origin_server_ts", progress_json))
+
+
+def run_upgrade(*args, **kwargs):
+    pass