Device list updates can cause tremendous federation traffic

*This issue was originally created by [**@kegsay**](https://github.com/kegsay) at .*

Because the update needs to go to all users who share a room with the user who changed device, this can lead to an EDU being sent to hundreds of servers. This spikes the goroutine count on the metrics. We should cap how many destination queues can be alive so at least this is bounded. Longer term we probably want to not do full mesh routing for fed traffic.