New class: kafka.consumer.fetcher.Fetcher
New class: kafka.consumer.fetcher.Fetcher

 - manages fetch requests via async client and subscription_state
 - based on upstream java client

Aloomaio/kafka-python,Aloomaio/kafka-python,gamechanger/kafka-python,dpkp/kafka-python,wikimedia/operations-debs-python-kafka,mumrah/kafka-python,scrapinghub/kafka-python,zackdever/kafka-python,Yelp/kafka-python,ohmu/kafka-python,gamechanger/kafka-python,scrapinghub/kafka-python,Yelp/kafka-python,wikimedia/operations-debs-python-kafka,zackdever/kafka-python,dpkp/kafka-python,ohmu/kafka-python,DataDog/kafka-python,mumrah/kafka-python