Set delivery callbacks per-message, not per-client

[cnweaver]

The librdkafa author suggests that callbacks should have bound into them the message with which they're associated, which greatly simplifies reacting to network disruption and ensuring mesage delivery, but adc currently forces the use of a single callback for all messages sent with a given Producer. This change relaxes that constraint, enabling full use of the underlying confluent-kafka feature.

[spenczar]

The code seems correct and reasonable. I'm happy to defer to your judgement on whether this change is desirable.

One concern is backwards-compatibility. Since this removes an attribute from the ProducerConfig class, callers will break if they used delivery_callback. But adc-streaming has very few (.... one?) users, so just not worrying about that seems fair, since you can probably be actually sure nothing breaks!

[cnweaver]

Naturally, for my own use the interface change is exactly what I wanted. To avoid breaking other users, it might be possible to keep the old way of specifying the callback for the whole object as a default to be used for invocations of the method which do not specify anything else? It might be a little difficult to express using no callback for a specific call when there is one set for the object, however, and I'm not sure whether the complexity would be worthwhile.

[spenczar]

Yep, I agree that the complexity there seems pretty high and it isn't clear to me either whether it's worth it.

[cnweaver]

Okay, in that case I have no further changes to propose, and consider this ready to merge from my side.