Make relays report bandwidth usage more often in test networks

[teor]

Let's put something more interesting in this line:

pc-rw-bytes 0

[teor]

These tasks are on the 0.4.1 roadmap for PrivCount and Sponsor V.

[teor]

Let's review these tickets at the next meeting using our 041-proposed process.

They're on the roadmap, so the review should focus on ticket size and team capacity (and sponsor expectations).

[teor]

These PrivCount tickets are on the 041 roadmap, we accepted their points estimates in 041 without discussion.

[teor]

This is part of an existing roadmap task.

[teor]

I have a patch, but it needs to be split up and re-tested after #29018 merges.

[teor]

I have incomplete branches for #29004, #29005, and #29019 here:
​https://github.com/teor2345/tor/tree/ticket29004-wip
​https://github.com/teor2345/tor/tree/ticket29005
​https://github.com/teor2345/tor/tree/ticket29019

I think all the necessary code is present in these branches.

But it needs some cleanup:

• rebase on to the current master,
• put the commits on the right branches
• make sure it does what these tickets say it should do

I'm happy to do that after I come back from leave. I am also happy if Nick wants to clean up this code.