Mark device list as stale, if we don't have the requesting device (#2728)

This hopefully makes E2EE chats a little bit more reliable by re-syncing
devices if we don't have the `requesting_device_id` in our database. (As
seen in
[Synapse](c52abc1cfd/synapse/handlers/devicemessage.py (L157-L201)))

syncapi/internal/keychange_test.go

@@ -22,6 +22,10 @@ var (
 
 type mockKeyAPI struct{}
 
+func (k *mockKeyAPI) PerformMarkAsStaleIfNeeded(ctx context.Context, req *keyapi.PerformMarkAsStaleRequest, res *struct{}) error {
+	return nil
+}
+
 func (k *mockKeyAPI) PerformUploadKeys(ctx context.Context, req *keyapi.PerformUploadKeysRequest, res *keyapi.PerformUploadKeysResponse) error {
 	return nil
 }