- Bump the version to v0.2.0.
- Introduce Cargo featurs: runtime-actix, runtime-async-std and runtime-tokio.
- Update the CI (GitHub Actions)

…o crate

- Replace the async async timers from tokio, async-std and actix-rt with
  the one from async-io.
- Change the feature names from "runtime-*" to "future".
- Create simple integration tests using different async runtimes: tokio,
   async-std and actix-rt.
- Remove throttle_write_pace

…:Cache