Rust 2018 edition/stable deprecation: 'warning: trait objects without an explicit `dyn` are deprecated'

[dconnolly]

[hdevalence]

I think that most of these trait objects are in the networking code, which is using an older version of tokio. Since we want to be using newer tokio and probably want to refactor the code anyways, I think it probably makes the most sense to do the refactoring first, then circle back to fix whatever's left of this issue after.