WIP: refactor(*): use futures 0.3 & tokio 0.2

[Cogitri]

Currently fails with

error[E0277]: the trait bound `(dyn core::future::future::Future<Output = std::result::Result<serde_json::Value, serde_json::Value>> + std::marker::Send + 'static): std::marker::Unpin` is not satisfied
  --> src/frontend.rs:68:5
   |
65 | impl<F: Frontend + Send + Sized> Service for F {
   | ---------------------------------------------- in this `impl` item
...
68 |     type RequestFuture = Box<dyn Future<Output = Result<Self::T, Self::E>> + 'static + Send>;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::marker::Unpin` is not implemented for `(dyn core::future::future::Future<Output = std::result::Result<serde_json::Value, serde_json::Value>> + std::marker::Send + 'static)`
   | 
  ::: src/protocol/server.rs:18:10
   |
18 |     type RequestFuture: IntoStaticFuture;
   |          ------------- associated type defined here
   |
   = note: required because of the requirements on the impl of `core::future::future::Future` for `std::boxed::Box<(dyn core::future::future::Future<Output = std::result::Result<serde_json::Value, serde_json::Value>> + std::marker::Send + 'static)>`
   = note: required because of the requirements on the impl of `protocol::server::IntoStaticFuture` for `std::boxed::Box<(dyn core::future::future::Future<Output = std::result::Result<serde_json::Value, serde_json::Value>> + std::marker::Send + 'static)>`