client: use tokio channels (#999)

* client: use tokio channels

This PR replaces the future channels with tokio because the APIs fit our use-cases better.

* remove unused code

* fix wasm build

* fix docs

* fix tests

* fix more nits

* Update core/src/client/async_client/mod.rs

* Update core/src/client/async_client/mod.rs

* fix unwrap