Understand why downloads are stalling in simulators, and fix it somehow
In #5336 (moved) and #4486 (moved) Rob reports a significant fraction of failed downloads now that he's reduced the capacity of relays.
Do these happen because of some Tor bug? Or some simulation bug?
It is especially important to find the answer because a client that stops getting bytes but doesn't get an end cell will stop putting load on the network, thus influencing the rest of the simulation.