* Introduce Group interface and AllGroup
As we move towards consumer groups we'll need to support the current
"consume all shards" strategy, and setup the codebase for the
anticipated "consume balanced shards."
The previous pipeline model required a lot of setup and abstracted away
the processing of records. By passing a HandlerFunc to the consumer we
keep the business logic of processing of records closer to the use of
the consumer.
* Add refactoring note and SHA to README
