Slower federation warm-up (#2320)

* Wake destination queues gradually, rather than all at once

* Delay device list updates too

* Maximum two minute warmup period

keyserver/internal/device_list_update.go

@@ -157,8 +157,15 @@ func (u *DeviceListUpdater) Start() error {
 	if err != nil {
 		return err
 	}
+	offset, step := time.Second*10, time.Second
+	if max := len(staleLists); max > 120 {
+		step = (time.Second * 120) / time.Duration(max)
+	}
 	for _, userID := range staleLists {
-		u.notifyWorkers(userID)
+		time.AfterFunc(offset, func() {
+			u.notifyWorkers(userID)
+		})
+		offset += step
 	}
 	return nil
 }