Add single connection support to Server

The `src/asynchronous/server.rs` currently assumes that there is a stream of incoming connections. For my scenario (NRI plugin) this is disadvantageous because I am already connected. Wrapping the connection into a Listener. with a `once` stream requires `'static`.

There is also the downside that `spawn_connection_handler` hides potential errors:

```rust
spawn(async move {
conn.run()
.await
.map_err(|e| {
trace!("connection run error. {}", e);
})
.ok();
});
```
#292 proposes a solution by creating a `start_connected` method.