Sbws should pin exits (or Single Onions) that have larger windows/congestion control

Sbws is not assigning stream ratios above ~2.2 to any relays right now. This means that it is not measuring any relays as faster than 2.2 times than the network average stream capacity. This is impacting spare capacity load balancing of fast relays.

The reason for this is because of tor's current 1000 cell circuit window and 500 cell stream window. These windows impose a speed limit on sbws stream tests that is dependent on latency. It both means that we will currently measure relays with less latency to the sbws exit(s) as faster, as well as be unable to measure their full spare capacity.

If we run a custom exit with a larger congestion window and stream window, then we will be able to measure relays with more spare capacity than current. We could then pin sbws to use only that exit.

Longer term, we should pin sbws to only use exits that have upgraded to support https://gitlab.torproject.org/tpo/core/torspec/-/blob/main/proposals/324-rtt-congestion-control.txt