Add support for MSC2697: Dehydrated devices (#8380)

This allows a user to store an offline device on the server and
then restore it at a subsequent login.

synapse/storage/databases/main/end_to_end_keys.py

@@ -1,7 +1,7 @@
 # -*- coding: utf-8 -*-
 # Copyright 2015, 2016 OpenMarket Ltd
 # Copyright 2019 New Vector Ltd
-# Copyright 2019 The Matrix.org Foundation C.I.C.
+# Copyright 2019,2020 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.
@@ -844,6 +844,11 @@ class EndToEndKeyStore(EndToEndKeyWorkerStore, SQLBaseStore):
             self._invalidate_cache_and_stream(
                 txn, self.count_e2e_one_time_keys, (user_id, device_id)
             )
+            self.db_pool.simple_delete_txn(
+                txn,
+                table="dehydrated_devices",
+                keyvalues={"user_id": user_id, "device_id": device_id},
+            )
             self.db_pool.simple_delete_txn(
                 txn,
                 table="e2e_fallback_keys_json",