Add an optional response callback to Producer

[bpot]

Adds a callback that can be used to determine which messages failed to send and why.

[kerinin]

Seems cool. Might be worth documenting a use-case somewhere

[bpot]

@kerinin true. I'll add it to the top-level Producer example

[bpot]

@kerinin @atharrison There is also another route that we could go which is providing a future-like object which the user would iterate through once completed.

[coveralls]

Coverage decreased (-0.13%) when pulling d29f06996cff33a5563186f476378a7d417ed577 on bp/response_callback into abbd3dfa13f1a341440d81cc4bdc38f739a57f05 on master.

[kerinin]

@bpot returning an iterator could get complicated with the retries - this seems like a more straightforward approach.

[atharrison]

Having a Callback seems more Ruby-esque, while having a Future that you block on reading the results from feels more 'concurrent'.

[bpot]

Cool. Just realized this design didn't consider errors that occur before we send the request (like not being able to connect to a leader). I don't know if it makes sense to overload the ProduceResult for that case or pass a second exception? value to the block.

[coveralls]

Coverage decreased (-0.01%) when pulling c703749aa512d5e8be7c48a48f335cd10f680be3 on bp/response_callback into abbd3dfa13f1a341440d81cc4bdc38f739a57f05 on master.