SlimMessageBus async api

[saithis]

Same as saunter (is uses saunter) -> blank page.

Also:

• Didn't get consumer without preexisting exchange working. Maybe commands where the consumer is the owner are not supported?
• Saunter didn't like multiple messages in one topic exchange: System.ArgumentException: An item with the same key and with an existing publish operation has already been added to the channel collection.