Remove the `async-trait` dependency

**Is your feature request related to a problem? Please describe.**
People who use this crate may not want to import `async-trait`. And we also don't really need it.

**Describe the solution you'd like**
Remove this crate, and use `impl Future<Output = ...> + Send`.

**Describe alternatives you've considered**

**Additional context**
I can do these changes.
