Add reply related methods for AsyncBroker interface

[s3rius]

As said in #87, we want to adjust result backends so they can listen to new messages on "client-side".

Here I want to add two methods replies_startup and replies_listen for AsyncBroker.

The first one would be an async method to initialize all necessary connections and things. And another method that would be used to actually listen to incoming replies.

Also here we can discuss whether we want to change names or logic.