Deprecate channel based consumer

[eran-levy]

Fixes #545

There are still few tests such as the consumer_performance_test which are using the channel based consumer but I will create another issue to address that specifically.

[ghost]

@confluentinc It looks like @eran-levy just signed our Contributor License Agreement. 👍

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clabot

[eran-levy]

it looks like there are some issues with the tests that arent relevant to this PR specifically, created this issue to address that #548

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[eran-levy]

@edenhill would you like to schedule this full deprecation for 1.9 as well?

[milindl]

Thanks for this PR! Closing this with #894 , we were able to deprecate channel based consumers.