This allows using a future `Executor` other than a `Handle` to execute the background (connection) tasks needed for sending requests and responses. This also deprecates `Client::handle()`, since the executor may not be a `Handle`.
This allows using a future `Executor` other than a `Handle` to execute the background (connection) tasks needed for sending requests and responses. This also deprecates `Client::handle()`, since the executor may not be a `Handle`.