Make 'Device list doesn't change if remote server is down' pass (#1268)

- As a last resort, query the DB when exhausting all possible remote query
  endpoints, but keep the field in `failures` so clients can detect that this
  is stale data.
- Unblock `DeviceListUpdater.Update` on failures rather than timing out.
- Use a mutex when writing directly to `res`, not just for failures.

keyserver/internal/device_list_update.go

@@ -342,10 +342,12 @@ func (u *DeviceListUpdater) processServer(serverName gomatrixserverlib.ServerNam
 		if err != nil {
 			logger.WithError(err).WithField("user_id", userID).Error("fetched device list but failed to store/emit it")
 			hasFailures = true
-		} else {
-			u.clearChannel(userID)
 		}
 	}
+	for _, userID := range userIDs {
+		// always clear the channel to unblock Update calls regardless of success/failure
+		u.clearChannel(userID)
+	}
 	return hasFailures
 }