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!