Inconsistent consumer/producer interface for different messagebus backends

[viktormelker]

The backends KafkaAvroBackend and MockBackend are inconsistent in how they are set up.

In KafkaAvroBackend:

• self.producer is an instance of AvroProducer
• self.consumer is a partial that can instantiate an AvroConsumer
  In MockBackend:
• self.producer is an instance of a (Mock) Producer
• self.consumer is an instance of a (Mock) Consumer

It would be good to keep consistency by:

• Making sure that self.consumer is the same "type" for both backends
• Making sure that self.consumer and self.producer are both instances or functions. This is not necessary but I think it makes sense and makes it easier for developers since you don't have to keep track of which was implemented in which way.

I suggest that we change self.consumer to always be an instance