Debug formatting could be improved

[gretchenfrage]

For example, when trying to print a quinn::Endpoint, when it gets to recv_buf, it just prints a list of many many integers with little discernible meaning. Possibly would be worth looking for a crate that enhances derive debugging with the ability to attribute structs with fields to ignore rather than doing it manually.

[djc]

I don't think we want to drag in an additional dependency for this. But having a manual Debug impl that's more discerning than whatever we've got now would probably be helpful!

[gretchenfrage]

The thing about recv_buf specifically was fixed in #2081. Beyond that, I figure this issue is probably too broad to ever say "we're done", so I'm going to close it, and just say, we should fix debug impls when we notice that one is bad.