Add callback-based method in Channel

E.g. in `Channel`:
```java
void asyncRpc(Method method, CommandCallback callback) throws IOException;
```

With:
```java
public interface CommandCallback {
	void handle(Command response);
}
```

This could allow to build a reactive API and also, combined with NIO, to have a truly non-blocking client (right now RPC calls are serialized, at least at the channel level).
