tor-netdir: Try to abolish unqualified uses of idx
tor-netdir has many kinds of index. Things called idx
are very confusing. Rename them all, to a qualified version of the name ("something index") or to some other name. And be somewhat more consistent.
This makes things marginally longer in some places. I think this is worthwhile.
Happily I think none of this is in the crate-public API.