tor-proto: remove `use asynchronous_codec as futures_codec`

The tor-proto crate has use asynchronous_codec as futures_codec in a few places. But if you're like me and don't notice this (like when looking at an MR diff), then it's quite confusing when looking at types like futures_codec::Decoder since you'd think it's referring to https://docs.rs/futures_codec/0.4.1/futures_codec/trait.Decoder.html. (I'm guessing Arti replaced futures_codec with asynchronous_codec at some point.)

I'm just making an issue and not changing it now since it would conflict with other current MRs.