Use aiokafka in rest proxy code

[gabriel-tincu]

Related to #30. All that's needed is switching consumers with async consumers. Admin client can remain the same until there's an async one in sight

Edit:
As of 2.01 ( 40c557f ) the consumers and producers are using aiokafka. The missing piece is the admin interface. It is an open question if the admin interface is a priority for the aiokafka project aio-libs/aiokafka#623 , aio-libs/aiokafka#665 (comment) aio-libs/aiokafka#639

[jjaakola-aiven]

Obsolete, the REST proxy uses confluent-kafka-python.