Model some slow background clients

For the simulation plan oomkiller tuning (https://gitlab.torproject.org/mikeperry/tor/-/blob/cc_shadow_experiments_v2/SHADOW_EXPERIMENTS.txt#L584), we are going to want to model some background clients that have slow and/or lossy links to their guards, and make sure that we don't tune to oomkiller so tight that it kills them.

I think that we can just reduce the rate of one or two of the markovclients in the shadow.conf.yaml file, but it might be nice to try a couple speeds of slow links, and a couple with a slow links + some percent packet loss, if possible.

Then, we can check the logs for any bad CC behavior, and make sure the oomkiller is also never triggered, with these clients.

This does mean that these clients will have to at least occasionally download some large files. If there is some probability that a markovclient won't do this, maybe we need some other approach. Or just have more of them?