Reduce the number of tokio-* dependencies

[tomtau]

The default features of the tokio crate bring quite a few tokio-* crates that are most likely unused (e.g. tokio-fs). It'll be good to reduce the number of these crates with a more explicit dependency in Cargo.toml:

tokio = { version = "0.1", default-features = false, features = ["codec", ....] }

where in features, one would only list the used / needed features of tokio.