Allow passing extra args to GenConsumer.init/3

[mtrudel]

Allow for extra arguments to be passed in as part of GenConsumer initialization, to avoid race conditions inherent in passing initial state information via handle_call.

The value of the extra_consumer_args option passed into GenConsumer.start_link/5 is passed to GenConsumer.init/3 as its third argument. The default implementation of this function ignores this parameter and simply calls GenConsumer.init/2 to ensure backwards compatibility.

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[joshuawscott]

👍 looks good to me; works with existing apps without any warnings that I saw.